Never used them myself, so this is just an idea to try....
Proof flyfishing, a rod building site/store, sells foam pads shaped to fit under the reel seat. He calls them "reel-tite" pads. They are in the slide band reel seat section. The idea is that the foam compresses when the slide bands are tightend down, and the back pressure helps keep the reel in place. I'd send a link but not sure it's kosher.