I built my own canopy; it's really a snap.
How tall you go really depends on the lighting you put in there. I did T5s and my canopy is 8" in total, I believe. It's a little taller than I wanted, but I haven't got a table-saw to rip long pieces.
I would recommend building a hinge into the front half of the canopy so that you can feed/mess with stuff without taking the whole thing off. If you want to see pics of mine (a quick job with sh**y tools that turned out alright), I can try to take some.
edit: *d out my vulgarity