First, dont listen to anything that
@Velcro tells you. He has no idea what he's doing

(He's got most of my acros at the moment)
Second, something to think about - I would seriously think about cutting the drain/return holes in the stand much larger and a single hole. I have big hands, so its hard to get hands or tools around the bulkheads if you have such a tiny space. Unless you are thinking about going bare bottom completely, I'd suggest cutting that hole big enough to fit both lines and a wrench. It wont wreck the aesthetics of anything, and will prevent some creative swearing down the road.
Third, I like the idea of the MH in the canopy, but I'd be worried about the heat build up. Depending on where the MH fixture vents from, you could easily pop a hole in the back of the canopy and mount a cooling fan in there to draw in cool air (or vent out the hot air) which might help a bit. I dont think many MH fixtures vent on the top of them, so it might be worth a dry fit and check now while you dont have it all together.