Mmmm, I would think they would eventually "settle" or maybe even rot depending on the type rubber. What about a polyurethane or a piece or stock I always called poly pinko?!
For me I used the wooden blocks but cut up pieces of an old fire hose and used top and bottom. The hoses are rubber lined with several plys of stuff including a type of polyester on the outside that will not rot..... this is just for info/idea's.
