Dude. I made a grow cab out of a storage shed like that. I thought it was cool till I got a tent. Then I just felt stupid for not getting the tent in the first place. Hence my name. Stupid.
Homeboxes are really nice. But if you have your mind set on that, I would sugest covering the inside with panda plastic, and big fat weather striping for all around the door. Can filter 66 as long as the box is 26' wide. It sucks cause you need the big carbon filter , so you can get a tube fan thats 400cfm just to keep a box that small in the 70's. Assuming your using a light that is worth even doing this with. Just something to think about. Good luck