I recently finished hand-sewing a knife-pleat kilt from a lighter weight tartan (not sure of the exact weight). This one was pleated to stripe and I had enough material to make each pleat double-deep, that is, I pleated to every other stripe. That puts a LOT of material into the pleated area and helps a lighter-weight tartan "swing" more like a medium weight.

Although I didn't feel the need, you could also double the material n the under apron to provide more, err, coverage in the unpleated area.

Good luck and have fun with it! (Mine's shown here BTW)