I ordered my two sheets of 4'x10' 1/2" ACX Plywood today. I'm not going to insulate the wall. For the doors, I'm going to laminate 4'x4' sheets of 1/2" around the door area to double the thickness of the door surround and limit the opening to 30" wide. These double up areas will also serve as a shelf for the masonite ceiling. (ala ceiling before spar method). The spar will sit on top of the ceiling on top of the shelf.
I also checked out their 1x2 stock and am going to go with either Poplar ($.41/ft.) or Oak ($.66/ft.) With the lighter (1/2") sides, I realize the importance of firm, straight and square spars overhead.
Also....I'm building a 10' trailer on an 8' trailer. 1' on the front and 1' on the back will hang over. The underside of the overhangs will be built similar to the top, with spars. I'll have to play it by ear a bit.
Just got a digi cam so pictures to come.