[palmier]

lasenza.com has low waist boyleg shapewear in nude colour ,maybe they would be ok to sew in the skirt . i have a couple of costumes that have boy shorts sown in the skirt to keep it in place.
I havent worn shapewear undies but i saw those in the store( one of my favourite store) and they dont have big seems so i think they would be discreet enough under the skirt.
at less than 20$ they would be worth a try.
if the slits are really high you just make sure you make a couple of stiches so it doent show the short.