Hot take but wear underwear under your kilts ya nasty.
"but a true scotsman-"ah ah shut up shut UP. I know more about American girl dolls western European 18th and 19th c fashion than you do genius. Also my grandmother's maiden name was Scott, just to flex my credentials.
True, men did not wear anything we would recognize as underwear under the kilts at the height of their usage. But, at the time, nobody anywhere in Western Europe of any gender wore any such garment under anything. No underpants, no trunks, no briefs, no boxers, no drawers.
What they did wear to serve the functions of underwear(protecting the upper layers from your sweat/other fluids to make for easier laundering) was a long linen shirt or smock. Your bare cock and ass would not have been in direct contact with your kilt. This idea of not wearing any undergarment with a kilt or plaid is just a historic misconception.
So either wear an 18th c shirt with your kilt down to your knees, or wear some god damn boxers and stop farting and sweating directly into your wool kilt that can't exactly be thrown in the wash.