I just put a piece of panda across the floor. It's pretty thick stuff. I'm using smart pots and tiny suacers underneath and havent had any issues.
Light proofing the door has been an ongoing battle. I velcroed the border and attached the panda, and put two zippers in to give me a roll up panel. The red lines in post 18 are the zippers. Problem is the zipper fabric isn't light proof. I ended up hanging a 3x5 piece of black felt that I bought for 5 bucks at a craft store. Seems to have done the trick.
Mine setup got complicated because its in a basement closet that I a to subdivide. The closet is 6.5' high, 2.2' deep, and 11' long. I hung panda to make a dividing wall, and put passibe air intake, my electrical wire exits, and my inline ducting through that panda panel. Sealing up those exit points was pretty much an exercise in duct tape. For the passive aor intake I boutght a cheap 12x12" air filter, wrapped it in no-seeum netting, and place a flap of panda in front of it attached only at the top.
I ended up buying a lot of stuff I didn't end up using, but I went through a lot of trial and error. Thankfully the grow itself it super simple! Water, Jack's Classic, soil and light.