Phenolic resin will work for your application, as Donald suggested. Go to www.mcmaster.com and search under plastics for "garolite". It is a compressed paper/fabric material embedded with resin for a very rigid material. Circuit boards and film holder darkslides are usually made of phenolic resin. You can get it very thick. 1/8 and 1/4 inch thick will be rigid enough, at least in my experience. Work it like very dense wood. It is available in sheets, bars, and rods.

-Greg