4x5! You got it all laid out! Can ya tell me what CFM the hockey table fan is? I'd think it would be a 300 or better! Pressurized with the passive system you have! fresh-air from below,with the box being insulated of course!Buy a roll of ducting insulation it's even got a silver finish.silver tape to help keep the construction work being done!Use that foam celitex board for the walls.It comes in 4x8 sheets. No screws,nailing, sawing,hammering! razor knife.You can just score the board with ya knife on one side and fold it! Now you can get it in any scuttle-hole there is!Tape the score line back with 3 inch foil tape!Just some ideas!

Ga Grown