Hello, i seem to have the same problem. To solve the problem I put the sporran chain the closet fartherst hole (tighter I guess). I than put my kilt belt on ( I use the belt loops) than I put the sporran chain on top of the belt.
It seems to work fine for me. Since I have done this i do not have a lot of problems with bunching up. Since I my waist and hips are only off by 1 , i do need to use the belt loops and keep my belt very tight, So the kilt does not slide off.