I used self adhesive 10mm closed cell neoprene. I cut mine to fit and had no problems. I do recommend a very high quality tape for the joints. I used Gorilla. When I put the body back on it was a snug fit so any thicker and it wouldn't have worked.
Word of caution. Use some tape to stick the locking nut to the end of your speedo cable, you may not notice if it drops down inside your chassis until its too late and then it really is too late. .