Build a rectangular box slightly larger than your tank top (so that it will slip over the tank), using 3/4" x 4" boards (A and B). Note that B is horizontal. This provides space in the back for hang-on filters, and for water and air hoses to pass through.
Attach short 3/4" x 3/4" square rods (C) to the inside of the box about 3/4" above the bottom of the A boards, and vertically from the B board so that the bottom of this rod reaches down to the same level as the bottom surfaces of the other rods. These rods will support the canopy on the tank top. (You may need to vary the distance from the bottom of the box to these rods, or add additional molding to the bottom of the box, to hide the waterline.)
Cut a lid out of 5/8" plywood (D), slightly larger than the outside dimension of the box. Attach 1" molding (E) to the front and two sides of the the lid to block light leakage.
Paint the inside of the box and bottom of the lid white to maximize light reflection (white paint is very close to a perfect reflector and is much simpler to apply than mirrored reflectors). Finish the other surfaces to your taste.
Attach lid to box with hinges. Attach bulb clips (F) to the inside of the lid (two locking clips or four non-locking clips per bulb). Place canopy on aquarium and add light bulbs and prewired waterproof end caps (G).*
*Note: Prewired waterproof endcaps can be purchased from your local fish shop.