I just went to the used parts store
http://www.halted.com/ and bought a piece of recycled metal. You could also choose a thick chunk of acrylic or something, too,
http://www.tapplastics.com/ Another good choice is wood; a bamboo cutting board is very hard and has a good tactile surface and texture that the mouse can read... Although if you're exceedingly cheap those grilling or smoking planks; they're often in yellow cedar which is soft and smells nice. ^-^
Personally, I just made my desk of redwood with a pull-out surface for my keyboard and trackball. I mean, why fool around with these things when I can get a chunk of rolled aluminum or copper cut to size?
-Crissa