While experiencing an extreme fit of 'matchy-matchy', I made a pair of stretch velvet undies to go with my brown stretch velvet outfit. Using Jan Bones' "Smoothie" pattern, and some scraps, along with 1/4 inch elastic and some powerdry for the inside crotch lining, I created this:
The only change I made to this pattern was to add a bit of extra fabric at the lower 'cheek' above each leg. One time, I was shopping in a lingerie boutique, and I queried the staff, asking them why so many underwear you buy rides up in the back. They explained to me that there has to be enough fabric under the butt-cheek, and it needs to be cupped under the butt with elastic. So, when I make underwear or bathing suit bottoms, I always add extra fabric in this one spot, and it always fits and is comfortable.