I'm going to build a solid wood box on all sides but the front face, on the front face, next to the door, I am going to use velcro and some dense fabric to make a front door that I can pull back. This will be good and light tight. I'm also not worried that it will get too hot in there because I will just be using one T5 fixture, 2' x 8 bulbs. Plus, I'll want it to be pretty warm in there since it will just be clones and babies vegging.
I'm excited to get this project underway! Got to try to keep my flowering plants happy though...which is going to be hard!