Here is
a plan for constructing a Basic DIY Aquarium Canopy. You can use plywood (1/2" or 5/8" treated works best) T-111 as in
Jordy's
300g Reef Tank, or any other wood (oak, birch or maple skinned plywood looks great) you like. Plywood sheets are 4' wide,
so if your tank is 48" wide (like your basic 55g tank), you can save yourself
a few saw cuts during construction.
Darryl
Craig's 180g Reef Tank is an example of what your canopy could look
like with a little work and not much money. Darryl used painted plywood
and stained trim on his canopy. Some other examples of DIY canopies which
may give you some ideas for yours: