Personally rather than buying a tent or building a box, just grab some tarp and tarp the floor, walls, ceiling and doorway. You can grab a tarp zipper to make a zip doorway. Much cleaner, easier to keep a stable environment; plus if you spill anything the place is protected. In addition it's a tad bit easier on the wallet, if you want to spend a bit extra you can use reflective sheeting that comes in 50 foot rolls instead of the tarp; which is what I have personally run. Most rolls go for $1.50 a foot or a whole roll for around 48 bucks.