Thanks for the pointers, JJ.
I made a bit more progress yesterday. I attached some foam blocks that I had layered up some time back to the sides of the extension frame and started shaping them to the contour/body lines of the bed:
I used a long block ($5 at harbor freight) with 36 grit paper to round them off to the shape of the bed:
Bottom, too:
And then started cutting the front to the shape of the bed-end:
Process is cut to shape of bed-end, slide panel forward, closer to truck, and repeat cutting to match shape of bed-end. Repeat until they're totally flush:
My blocks weren't quite thick enough to follow the entire curve of the bed, so I'm going to go back and glue another half inch onto them to shape.
To be legal, I think I'll need to do a yellow blinker on the side of each of these. I'd like to paint them gold to match the truck, once they're glassed, filled, and block sanded. The shell and everything on the back of the truck will be white.
The two things I'm thinking about right now are how I'm going to attach the fabric to the bed and to the bottom of the shell as well as how the shell lift will work... I need to figure these out so I can make any needed cuts to the bottom of the shell before I start glassing it... or I might be able to go ahead and glass the top and most of the sides of the shell, then make adjustments on the bottoms of each side, and then finish... hmm... braindump... :P