I have been thinking of getting a sheet of plywood, trimming it to fit, and then covering it with carpeting to drop into the back of my Ranger to give the bed a flat floor. It would accompany the truck cap I currently have in place but allow something a little more comfortable for camping, hauling stuff, etc.

Does anyone have advice on doing this? My thought was to use cardboard to make a template to then trace the outline of the wheel wells on the plywood before cutting.

 

Yes, that would work fine, not sure you need to cut out wheel wells, you won't be sleeping at the edges, so I would just cut a piece that slides in between wheel wells, easier to put in and pull out.

If you decides to do full side to side then I would do a Front and Back piece, to make it easier to get in and out, a rectangle piece for the front, cab to wheel wells, then inverted "T" to slide in between wheel wells.

Lifting out a 6ft piece of plywood with cap on can be hard