Polyurethane question

Since the hive knows all...

I bought a pair of polyurethane spare tire "bumpers" for my Wrangler. They look like this:

My stock bumpers are about 2" too short after putting different wheels and tires on. I thought I'd be clever by shoving a black racket ball in there. It has worked OK for about 3 years but the ball is egg shaped now.

Anyways, I bought these because they are longer and should work better than the crappy aftermarket foam ones. But, these are hollow. I thought they would be more solid (filled up?).

I've searched the web but keep getting results for filling up rubber engine mounts with polyurethane.

What can I use to fill these up and make them solid? Will 3M Window Weld stick to polyurethane?

They are probably supposed to compress a bit, not be solid.  I'd leave them hollow.

You need the air gap to get the crush to make them springs.  I would leave them alone.

+1 for not filling them unless there's a problem.

Buy some castable urethane from Mcmaster-Carr and fill up the hollow part, if that's what you want.

I would use this stuff.

http://firestonebpco.com/assets/2013/04/1303-pourable-sealer-s-10-09-17-12-ns.pdf

Shoe goo?

I would argue that since the actual question is not made of polyurethane, that the thread would more correctly titled "Question regarding polyurethane."

I'd leave them hollow if they are supposed to give a little.