Hey Mike,,the 1x4s are where shelf , bulkhead and other connections are going to be made,,the walls are sort of glue laminated panels,,,tempered hardboard on outside,,1x4 framing and blocking, and then a sheet of 1/8" plywood on the interior,,since the unit is 9'-2" frame length and 9'-6" bady length I had to do some splicing,,,to have a little more strength i used a 3/4" x 2' piece of plywood on the front and rear and framed in the middle,,then screwed and glued the outer skin on,,inner skin of 1/8 ply goes on next,,this is probably overkill but I didnt want pieces flying off going down the raod,,
as far as the basement hatch, ,think Ill just tip it up and use it as the other side,,,,

ya'll keep em straight
chip