Believe it or not I use a really big, but thin two-sided buckle. The circle of the buckle is the size of a big penannular, buit the metal itself is not very thick. I removed the tang of the buckle....the buckle is brass.

I just tuck the two ends of the great kilt through the spaces available in the buckle. It's not "period" but it looks OK and it works and it didn'tcost me anything at all since I already had the buckle.