my shell has a fiberglass interior so can't Velcro something onto the interior surface.
so the soln I ended up using was 3M adhesive hooks.. 3 on each side for my side windows that are 5 ft in length..
Then wrapped fabric around a skinny (but stiff) PVC pipe. Then just rested the pvc into the hooks. Then I used zip ties to make sure the the pipe doesn't fall out.