if wood, be sure to finish/seal bottom, or it will warp.
Much better than glass:
If odd, found, rough materials, use pour-on acrylic finish you can get at any hardware store. The instructions are great on the package. Use torch for co2 bubble popping and tent it with poly, etc. Full info here:
http://www.eti-usa.com/consum/envtex/envlite.htm
Instructions and tips links are right at the top of the page. This stuff is bulletproof. I love it. If you don't want high gloss then rub with scotchbrite pad to dull...