Ding ding ding! We have a winner! Downunder has it right. There's also a less-expensive version around. The Bandspec has snaps for the over-cape to be folded up out of the way for pipers when playing.

Basically any type of Inverness cape or longer overcoat - or even a long rain poncho, see your local camping supplier - would work if you want to protect your kilt from getting wet.