I ended up sleeping in this morning, so didn't make it out to the flying field. However, I did make it to the Big Box Home Improvement store for some plywood and hardware, and to Industrial Metal Supply for some scrap aluminum pieces. Using a 48" x 57.5" sheet of 15/32" CDX plywood, I built a shelf for inside my truck's shell, so I can carry more planes to the flying field.
My original plan was to have the long dimension across the shell, so that it would sit on the bottom frame of the shell. Unfortunately, with the width of the tailgate opening and the taper of the shell itself, I couldn't manipulate the plywood into the shell in that orientation. On to plan 'B', which involved using some of the aluminum channel as a cross bar in the shell, and other pieces of aluminum to act as fences along the edge of the plywood, stiffeners underneath, and stops to keep the plywood from shifting forward or back whenever the truck changes speed.
Two eyebolts for some bungee cords to stretch across landing gear, in order to help prevent planes from rolling around, and she should be ready for a trip to the SAGE field in the morning. This should allow me to carry at least 3 more planes, possibly more if I get creative in how I arrange them. The shelf is easily removable, if I need to haul something bulky in my truck.