Duke was... tough. There's not nearly as many differing designs to draw on for him, and frankly I don't like a whole lot of them. I do like the mainline Signal costumes I've seen (both the black symbol w/ full-face helmet, and the glowing symbol w/ jaw exposed) but they really have more "Independent Hero" vibes.
(These designs are all dope, but not great sidekick material.)
Digging through reference images led me to realize that even in his Elseworld appearances, Duke almost always gravitates towards those knightly vibes. Boy likes his armor with a little fantasy-flair.
("Future's End" & "White Knight" a bit less so, but I still get cyberpunk-knight vibes from both—maybe it’s just me.)
Those that aren't all knightly tend to be stylized but durable street clothes, that are just out there enough to pass for a costume but could just as easily be worn by someone in their everyday life.
(Last one is this redesign by @sufroyo, but gods do I love it!)
Duke will probably want some of that knightly vibe, but Moonbeam is kinda my Robin figure in this AU; he needs to look like the less-experienced partner to Gotham’s Dark Knight, rather than a Knight in his own right. And I should combine that with his preference for practical, durable, almost-understated fashion.
So… streetwear-squire. Let's see what I can do.
First off, we can get both sets of vibes by starting with a gambeson as our base. Go for a shorter one to emphasize that jacket look, and while they come in a whole variety of styles, I think off-center clasps looks just a little bit more modern. I'd also say he doesn't buckle up his neck, giving it that popped-collar look Duke seems to like.
(Off-centered clasps, as well as a couple casually worn gambesons to show the kind of effect you can get. Duke's is definitely the kind that covers the hip to brush the top of the thigh, but given most people think of gambesons as being knee-length, that's still short.)
And, hell, if Robin can wear bright colors then so can Moonbeam. Using the "It'll help you learn to sneak better" excuse, Duke's gambeson is cream colored. (It also looks better contrasted to Batman's black.)
I like the idea of the gambeson having built-in gloves like in Duke's "Final Knight" costume, but I don't really like the look of Final Knight’s Duke design(s). Erm, any of them. I'd define the difference on the sleeves a little more by giving him hand & arm bracers to wear over the top, probably some simple pauldrons, and maybe a gradient on the arms from cream to grey.
(Not 100% on the gradient, just think the fully light sleeve might not work even with the bracers. Keep the metal simple & understated, though, and remember the lines in the sleeves are from padded fabric rather than any kind of wrappings.)
Still, that's a long stretch of one pretty plain color. The main body needs to be broken up, and Duke should probably have a bit more protection, but we don't want to hamper his mobility. Let's add a demi-brestplate (also called a demi-cuirass or "heroic armor") on top. That protects the upper chest without going past the rib-cage, to avoid hampering movement.
(I really, really like this fluted design, but can't find anything smiliar in a demi. I've also always liked the look of breastplates which are 2 pieces held together by straps under the arms. The last one is just to give you a better idea of the size/form of the thing.)
And a steel-grey utility belt, obviously.
(...I should probably mention, none of this metal should be highly-polished. It's not glinting like a mirror whenever Duke's not redirecting light away from him, it's just a step or two above matte-finishing.)
I'd say Duke's symbol is an iridescent/opalized circle right in the middle of his chest, representing a full moon (or the bat signal minus the bat.) So it looks white, but it shimmers rainbow when it catches the light. Probably has either a black or gold ring around it to help it stand out.
(A normal opal, and opalized glass with light shining through it. No, the ring does not seem to exist outside of pinterest.)
There's also a matching smaller "jewel" on the back of each gauntlet, and Duke claims he uses them to focus his light powers. Whether he does or not, I don't know.
I don't like the look of Duke's "We Are Robin" helmet in-context, but I think it could work here. Match the grey to the breastplate & gauntlets, match the stripes to the gambeson, replace the eye-slits with a thin, one-way visor, lose the ear-circles.
(I did consider having the colors be the other way around, but that felt a little too close to Ghostmaker's look for my liking.)
Give him some dark grey cargo pants, a pair of these sports boots (minus the logos), and some metal knee-pads to finish the look.
A handful of these characters I do actually have a degree of costume evolution in mind beyond just changing identities; Duke is one of them. In this case, it becomes more knightly as time goes on.
In particular, I think Duke swaps the gambeson for a chainmail tunic and the demi-breastplate for a laminar chest piece.
(I like the short sleeve tunic style, and I'm picturing Duke's "White Knight" armor as the basis for the chest piece. I can’t seem to find any laminar reference photos that have that same look; I’m almost picturing a scapular, but made from metal plates.)
The sports boots become combat boots, the knee-pads & bracers get swapped for splint greaves & splint bracers over black leather gloves. The “jewels” are now on the back of the gloves directly, rather than the metal, as the new bracers don’t cover his hands.
He also adds the Bat ear-points to his helmet, adds a dark undersuit to the top to replace the sleeves, and generally ups the contrast by darkening the greys & either polishing the armor or painting it metallic white (if the latter, the chainmail is probably a light, brassy brown to make them pop.)
(These colors, I believe would depend on the artist; it seems like one of those things people would disagree on.)
All of which helps evolve the look toward the Signal's "motorcycle gear meets knight armor" mash-up.
I've seen close to 4 seasons of SCTV, and I think I know what they should probably do (or what I would do) if they did a reunion special: I'd want to do a Mockumentary film or miniseries. A where are they now type thing. Since I think I've heard the show ended with SCTV going off the air because they didn't have enough money, I think it'd make sense to do a fake documentary covering what happened to all their many stars. I could still see a comedic tribute to John Candy and his characters being written in, because I could see Johnny LaRue dying in some embarrassing way (like David Carradine).
I really think I'm onto something here. At the very least, it seems like a fun idea. Who wouldn't want to see where SCTV's many personalities ended up?