oh, almost forgot. I went with .5" ply inside and out. That with the 2x4"s will make the walls 2.5" of nearly solid wood. When I screw down the ply I am basically making a grid out of the screws. I am putting them every 6". So it basically makes six inch squares over the ply out of screws. if you do the math that's about 128 2" screws per sheet of ply wood. using roughly 20 sheets of ply, that comes out to around 2560 screws for just the ply. I've already used over 4000 screws total. When it's all said and done it will have over 35 lbs of screws alone. Not to mention about 15 tubes of subfloor adhessive. The ply in the front where the viewing windows will be is going to be doubled up so that it will creat a shelf for the windows to sit in. So the front wall although it will not have any ply on the front will have 3 sheets on the inside, plus the 2x4s on the outside. Comes out to 3" thick front wall. I'll keep you guys updated, but not sure how much progress I'll get done this weekend.