You could do worse than optical grade cast acrylic- it machines very well (don't use extruded, that doesn't machine nice). Get an optical company to tint it and stick a hard anti scratch coating on it- that'd work. Attached is a cube that I made from acrylic and tinted it, it's not polished, but you can get the idea of the tinting effect.
Hope this gives you a little help
Oh, RS catalouge sells the stuff here in the UK there's probably loads of sign companies that sell the stuff too.