I've done this a few times. It really works well.
I just build a box with no floor. I just take the measurement of the upper rim of the tank and add 1/4" to both the length and width.
So if the tank is 48"x20", for example, then I build the box so the base has an opening of 48.25" x 20.25".
Then I just attach a cleat on the inside of this box, usually about 2" or 3" up. This whole thing just sets right over the top of the aquarium.
I have only used 3/4" plywood for the box and strips of plywood for the cleat. The weight comes in handy for keeping it stable and secure. But I do believe there are fancier ways to do it, I just have not tried it.
I always cut the door openings out of one of the solid panels of plywood instead of making a separate face frame. You could build whatever door style suits you best.