It is always better to seal wood. In your case it does not matter how. You do not need to stain over the polyurethane. It will be fine. You just want the moisture to be kept from the wood. The shop light will rust but it will not matter all that much.
I am making a new canopy as well, is there any benefit to mounting mirrors on the underside of the canopy? Will it make a difference in the amount of light vs painting it white?
It's the frame that supports the weight (if you build it that way). I used luan plywood which is <1/4" thick.
Bmeasure, is yours hinged like this... http://community.webshots.com/photo/66733948/66735150QxwYEB
I really need to update these plans... http://community.webshots.com/photo/66733948/67041321sRbRzB
I've since removed the metal housing of the shoplights and fastened the endclips right to the frame of the hood.
Mirrors use glass over the reflective surface, even though it's minimal, there is light loss, more so than a painted surface...from what I've read. Check out thekrib.com.
Yes, for the most part the hinged door is like that one, but I have a solid top and front 3" in on both sides of the canopy. It has been more difficult to get things to fit correctly together. It isn't my measurements for the most part, it's the wood wanting to bend or cup (so slightly you can't see it until putting on the door) in certain directions. I'm not sure the face of the door is going to come out perfectly flush with the solid front parts. The top is looking pretty good, though. Either way, this is a difficult way to do things and I'm pretty happy with how well all my measurements and ideas have come together so far. I'm priming the inside of it today, so it shouldn't be much longer now. I still have to paint inside (2 coats), possibly seal with epoxy (thin coat), add all the trim to the outside, stain/poly exterior 2 or 3 coats, add black mesh screen to rear, then all the hardware. WHEW! Then I need to put in the lights! I'm going to be putting in 4x55w CF lights from AH Supply. I've really liked their lights so far.
Just wanted to add that I got the access door to fit much better now. It was the hinges holding the door. They are not completely flat when laid down (the hinge itself is rounded on bottom a bit). I ended up routing out the areas holding the hinges and things are almost perfectly flush now. I am really happy with how well this thing came out. Once I get this thing stained, sealed and add the hardware, it's gonna really tie the look of my tank together!