If you want an inspectable tank lid go with Acrylic. The thicker the better. Use soft compressible rubber gaskets and lots of screws around the perimeter. The thinner the material the more screws you need. Don't forget to make the screw holes slightly oversized and make sure they're all exactly centered so the expansion/contraction doesn't crack the acrylic.

http://www.rockler.com/rockler-insty-drive-self-centering-bits