I finished my slitted hemipenes packer! (Sorry for the shitty pictures, it's like 7 PM)
It's really shittily made because my main concern was function, but I covered my foam hemipenes in black pleather, then made a sort of "sheath" out of the sleeve of a sweatshirt that ~sorta~ matches my skintone, to act as the slit. Ideally I'd find a way to compress the foam better.
I just flipped the sleeve inside out once I cut it out with plenty of excess, sewed a gathering stitch, flipped it right side out, cut a hole at the end of the gathering stitch, stuffed the hemipenes in, and sewed a button onto the other end of that same side, and kinda just folded the rest of the fabric under itself to give it about the right shape. The loop the button makes will hold whatever I use to strap this to my body.