I use 1/8" birch plywood that I get from a local art supply store; most hobby shops carry it as well.

I've made boards for my Shen-Hao (the same as Technica boards), for my 2 Cambos, and for my Calumet C1. The wood's always been very stable, easy to work, and can be stained or painted as desired. And it's cheap, too...an $8 piece of 12" wide wood gets me about 6 or more lensboards depending on the cameras I'm working with. I use a box cutter and a Dremel tool for all of the shaping work, and wood glue if I need to combine pieces (as, for example, a C1 board). I think the longest it took me to figure out how to make a board (for the Cambos) was about 10 minutes. It really isn't rocket science...and you get the satisfaction of making something that really works. As an added bonus, the boards are generally lighter in weight than metal boards.