jags,
Could you fill the gap with a thin board? I'm thinking something like a piece of Formica countertop with a couple holes drilled in it, then zip-tied to the bag supports so the bag can't fall through? It wouldn't weigh much. Alternatively, some webbing straps with quick-release buckes would go far toward making a platform.
Just a minute...I read your post again more carefully...could you put a stiffener in the bottom of the bag before placing it on the rack? Kirtland used to do this with their panniers, using a piece of tempered sheet aluminum and wraping duct tape around the sharp edges. I did the same to make stiffeners for my own panniers and bags, using some discarded bread delivery-truck trays -- they were also made of tempered aluminum, and were made obsolete when the size of loaves changed (!). I picked up a few, and bitterly regretted not taking the lot, but storage was a problem at the time. Anyway, try a stiffener inside the bag to see if it helps, jags. That's what I'd try next.
All the best,
Dan.