Will you even be able to see the kilt under the robe? I'd just tie the robe and wear shorts underneath... easy enough. Or you could buy some cheap-o fabric from Wally world or somewhere and have a go at an X-Kilt. Im' sure you could get fabric for less than two way shipping of a loaner kilt would cost. And, if the X-Kilt didn't pan out to be the greatest creation ever: what's the harm? Can't really see it in your costume anyway. Sounds like you've got plenty of time too.