HTG is GREAT!!! They have cheaper light set-ups than that though. Also, you can buy 1inch by 1inch by 8ft boards at Home Depot or the like for about a dollar a board and screw together a frame that tall. Then, buy some tempered hardboard for the walls outside. Then just use Gorilla Tape on all the light/air seems. The hardboard can be cut to size at the hardware store and any further holes for lights or fans can easily be cut with a variety of hand tools. Tempered hardboard is only a couple dollars for a big sheet. So, with those materials, you would have a very durable box that size for probably under 15 bucks. I used the hardboard to cover a window. It is thin, and CHEAP! I can not think of a cheaper box, that is still worth the effort. You could use one like the above for years. It would also be very lightweight and could be lifted from atop the plants so there would be no need to move the plants in or out. Good Luck!
-Dude