It's an age old question and different materials can be more suited to different patterns. It is tough to beat wood for all around properties, workability, and economy. REN Board is just closed cell urethane which can be bought in all densities, even lower densities than EPS insulation board, but it is very expensive.

One thing that wasn't mentioned above is the machinable countertop materials. Corian is a DuPont trade name but it's a filled acrylic that is reasonably free machining to the point it can be machined with wood working tools and worked to very fine finishes with sand paper. It's a harder material and harder to work than most woods but also more dimensionally stable. It can be glued together with epoxies and if the surface is scuffed, filled and detailed with polyester (bondo).

It used to be all the rage for high end counter tops and is still used. It's probably too expensive to buy new, comparable to the urethane tooling, but it is old enough now that a lot of it is being ripped out and replaced with granite in high end installs so if you befriend a counter top guy, you might get it for free.

Another pattern material can be Plaster of Paris. It's dirt cheap but you typically need to cast it and usually seal and finish the surface. Filling it with glass fiber or hemp makes it much stronger but when dried it can be sanded and to a limited extent carved.