I've seen eggcrate used as diy aquarium tops, as platforms for viquariums, etc., all over sites, but I don't know where you can get it. Is it available at hardware stores?

IF by eggcrate, you mean the white (usually) plastic grid (1/2 inch square grid) that is often used for ceiling lighting covers, yes - it is sold in hardware stores. Look for it in the lighting department - you can buy large sheets of it and cut it with a small toothed saw to fit the way you wish it to. I also cut it to fit as dividers in a tank - it makes a great divider. I fasten it into the tank with dobs of sealant - easily peeled out if and when I wish, but permanent in a bare bottomed tank. It also is strong enough to make a top for a tank - but offers no protection between a light and the water - the water will still condense on the bottom of your light. You should have a more water resisitant barrier between the light and the water.....

Thanks Daryl!