polyurethane sealer. Same stuff you woudl use for a nice piece of wood Furniture. I stained the wood to the desired color and then applied 3 or 4 coats of the poly. applied a bit thick in some areas and have a few runs (my fault), but since sealing it, I dont have any flex in the wood. for the top, I used a 16" pine cabinet board that was basically several 2x1" pieces pressed and then glued together and a 4" pine board with a piano hinge in between for the top. when I placed on the tank naked (no stain or sealant), it would flex and seperate about 1/2" on one side. I braced it with 4 pieces of aluminum angle and 1x2 pine that ran from front to back and were spaced evenly left to right, but still had a bit of warping. just before I got my lights, I stained and sealed it and after that it flattened out, and you can barely see any warp, and it fits flush around the entire rim. it flexed so much before it started pulling some of the 1.5" screws out.
For my 75, which is 48x18x18, I used 16x1 pine board on the sides, and then the top described above using part of the scraps from the sides to do the non moveable portion of the sides. I used 8" pieces of 1x2" pine in the corners for support and that is all the lid uses to sit on the tank top. I then cut out some rectangular doors in the front and cut most of the back board out for all my plumbing and cooling fans.
when i put my lights in, I had spaced the alum braces just right so that the endcaps mounted to the bottom of hte outside braces and was able to route my wires down and out to keep them neat.