Google “making rubber.” There are a lot of kits that are two part, mix and set. The last time I looked the company sold three hardnesses. The bad news was that it was about $80-$100, only came in gallon jugs (which was about ten times the quantity I needed.)
Ive also wondered if that flex seal stuff would work. The ad that shows them pulling it out of a box looks like it’s rubber. No idea it’s density, though.
https://www.flexsealproducts.c...-rubberized-coating/