I put everything in an oven pan then used a LCD screen diffusor panel for the top. I drilled holes in the side of the pan and sealed them with caulk. I did eventually get some water ingress from daily pressure/temp changes + humidity and significant rusting of the bottom of the pan but the elevated wrt54g was operational (if a bit nasty looking) when I took it out after 3 years. If I did it again I'd add some weep/drainage holes to mitigate build up.

http://a.superkuh.com/wrt54g-in-oven-pan-with-caulk.jpg