I'm looking on ebay for some plastic rod. I measured the shock hat ID at 4.1 inches so I'll look for 4.5 or larger diameter bar stock. I would assume Delrin would be best but is there any plastic I should stay away from like PVC?

 

Anything that is really brittle you want to stay away from. Look for impact strength, if poor stay away from it. Go to mcmaster or similar sites selling plastics and it will have characteristics with each. That will give you idea.

 

ASTs adapter for use with the stock hats sounds like exactly what you need or something similar. They have a hard plastic piece that goes in the stock top hat after removing the rubber and it keeps the spring in place from the outside not by a ring on the Inside. It sounds like exactly what you're looking for I just don't know that you can buy it individually.

 

How about going to Home Depot and buying some abs plastic rings. This very strong

 

You'd have to secure the ring in the hat some way. We're talking about making a spring cap that the springs ride on to hold it in place.