do a google for 'DIY
aquarium canopy' and you'll get a lot of resources.
i'm not too handy, so I just build a 3 or 4 sided box, then attach a top piece, hinges, and then the flip open lid section. then just put some blocks inside so it can rest on the edge properly, and you're good.
definitely used a topcoat sealer, or an oil based paint on the interior to prevent moisture from getting in and warping the boards.