I have been making very limited numbers of UnderKilts for years now.

Here is how I do them.

Take a new t-shirt that has a chest measurement larger than your hip measurement.

Cut if off just below the armpits.

Sew a length of elastic to the now top opening. Just like the waistband on regular Jockey shorts.

Pull the t-shirt on like a ladies slip.

Don the Kilt. Voila. The UnderKilt!!!!

I think I can make one in about 20 min. The length of a t-shirt is almost perfect. If it is a little long roll the waistband up once or twice till you get the length worked out and then re-stitch it.