Space Age Paint to wander their aisle of rubber seals (available by the foot,) looking for something to replace the stock seal. This is what I selected. It's a GM roof rail seal but more importantly has the right dimensions, and has a smooth, tough, outer skin which I hope will stand up to wear better than the original foam rubber.
Space Age Paint has a small web presence that does not support ordering online. Jeff tells me that they will do mail order, and they sell by the foot instead of just by the carton, so here's his contact info. If you're in the desert southwest, you can stop by, but don't bring your trailer. Their parking lot is VERY unfriendly for trailers!
I have no relationship with Space Age Paint - I just appreciate the weird and wonderful stuff that I can find there!