I have lengths of dark grey felt from the fabric store, self-stick squares of hook-side velcro stuck to the window frames in strategic spots. The felt sticks right to it. The velcro glue will fail after three hot summers or so, just stick more on. I used to sew (or just staple!) the hook-side to the felt, and self-stick it to the shell liner. The liner starts to fray and look terrible and not stick after a while. I used to take them down but now just leave them on at all times. When I need lots of ventilation while sleeping I roll them out of the way.
I have a ton of backpacking thermarests, I keep those in the bottom of the bed. Then I keep a foam mattress from IKEA (Beddinge - it so happened that I bought one to use as a folding couch/bed, got a different replacement mattress, repurposed the old one to the truck) on top of those. It comes in a washable zip-up cotton cover, folding lengthwise at the middle, with two foam panels inside. I removed the foam panels, test fitted them and cut them to fit my Tacoma bed with a breadknife and stuffed them back in. So there's foam corner to corner in the tailgate end where my head is - very comfortable. In the front of the bed, I left the corners free, just the right size for an ammo can each (compressor, recovery gear). When you need cargo space, it folds in half and can be stored upright and nearly out of the way.
I made a shelf at the front end, sits on top of the shell rail, half the length of my 6' bed. Sleep with feet under, tupperware containers on top lashed with rafting cam straps. The shelves are Home Depot carpet stapled onto 2x10s cut just to length. Huge patches of hook-side velcro are stapled onto the bottoms and hold the shelves in place to the shell liner. Or rig an L-bracket and bolts to the bed rails.