I opted to keep the original aluminum seats in to help with rigidity and just covered them with plywood and carpet.  Side walls are 3/8” a/c plywood sealed with deck seal, found it on the clearance rack at Lowe's (wrong color mix and was discounted from $22to $6).   All expenses are listed below. 

Floor is again ¾” plywood and did not attach it, it actually fits between the side walls pretty snug and I can easily remove it to clean out.  Rear deck is supported by the original bench seat and the rear deck hatch closes against the transom.  Use ¾” wood for the rear hatch to support some one walking on it.  All of the wire in was covered with wire loom where it exited the rigid conduit.  The boat used to be light enough to pull around by hand, but now I had to put a tongue jack on it to save my back.