Thank you, Lee.
I formed the back of the cab by laminating a couple of pieces of sheet plastic and gluing them to the cut behind the "B" pillars of the PT. (I used the original issue PT, not the convertible, by the way.)
Next, I used sheet plastic to form the bed floor on top of the chassis. I found the wheel wells in the parts box (don't know what they came from). I fitted the body to the chassis with the floor and wheel wells in place and used card stock to develop patterns for the bed walls. Then used the patterns to cut thin sheet plastic. After heating the single piece in hot water, I formed into a "U" shape that conformed to the cargo area opening.
I tacked it all together with Tenax, and then used epoxy to blend the joints and corners together, forming a uniform radius all around the interior of the bed. Once it was set, I Tenax-ed the upper edges to the PT body and cab wall, and finished it off with a slight rolled edge. Lastly, I applied the 'spray-in bedliner' texture.
Then I built and stained the wood skid, marine-style, and dropped it onto the bed floor to visually break up the blackness of the bed and give it some interest.
Hope this helps. It was my first attempt and I thought it turned out pretty well, all things considered.