Hey Chris,
A canopy is one of the easiest things to make for an aquarium. Measure the outside dimensions, and make a three sided box with those dimensions inside (so that the box fits all around the outside of the aquarium). All you'd have to do is add your board thickness to the dimensions of the tank. Make the front the longest piece, so that the seams aren't obvious.
Cut a 1 x 1 to the height of the boards minus, say, an inch and a half or so. Use this on the inside corners, and it'll make attaching the front and side boards way easier. Measure carefully so that the bottom of the 1 x 1 is a consistent height from the bottom. Cut a few more pieces, and attach them at this same height (1.5" or so) horizontally. They'll support the canopy on the edge of the tank. You'll want to position them so they don't interfere with the lids, of course.
Lastly, given the length of the tank, you'll probably want to cut one more 1 x 1 that goes from one side to the other, up top, to keep the sides from flailing out.
Then stain it and add any decorative elements you want. Super easy.