That sucks. The bag is really designed to have the weight rested on the bottom, the top straps don't do too much.
If it is difficult to close, it seems like the top tube would be supporting some of the weight, but that's a bad area to put stress.
For your frame, maybe you should make a supporting bar of some kind. I've had good results with my trek, probably because the bottom tube is sorta flat, preventing the left/right problem.
There are some other triangle bags out there, but they aren't as big. They would still warrant some investigation. I already wasted $$$ on the power in motion / crystalyte bag, so you guys get to be the guinea pigs for the other packs..