i'm building one myself this week. let me know how you do.
i'm not sure what kind of design you're doing, but mine is about 5" higher than the top of the tank, with a cutout for a 24" compact flor. striplight. (the tank is 30")
I'm looking at building a basic frame work a couple of mm larger than the tank. To this I'm going to screw some hardboard with some 1cm baton about and inch up from the bottom to rest on the top of the tank, this should give enough space at the top to fit the light. I'll enclose all the cabelling inside the hood in plastic trunking and probably screw a switch box onto the back to power the air pump, filter, light and heater. Havent looked at light units yet so I'm not sure if I can buy reflectors or whether I'm going to need to make some. The outside I was toying with idea of buying corner batons to cover the joints and the either paint, stain or varnish it.
Shouldn't take too long to make at least with this way adding holes for tubing etc should be a doddle.
I'm planning to build one for my 4ft tank (it's a non-standard size and only has one light tube built-in - a 25 watt, nowhere near big enough for my plants). My cunning plan is to build a frame out of ply wood struts and clad it in plywood. I also intend to build a condensation tray into the floor of the hood.
I've been known to slink around my local fish shop with a tape measure and sketch pad . I look closely at the furniture that they have for sale and make a list of hardware, measurements, etc.
Thinking about it thin ply wouldn't absorb as much moisture as hardboard so is probably the way to go.
Also trying to figure out whether to build a stand for it or not, thing is if I build a stand big enough I might be able to find another tank to go underneath it... hospital/quarantine tank only you understand