I had the same problems sitting with a rigid backed sporran. I don't know what kind your using but I solved most of the problem by getting a softer sporran which bends with my body when I sit. Now I only consider wearing a rigid backed sporran when I need something formal/semi-formal and it's not going to be with me that long. I figure why wear a more comfortable kilt only to be assaulted by an uncomfortable sporran?

Cliff