A few years ago when I started getting into camping/ overlanding, I decided to build a sleeping platform so I could be more comfortable. I had more money than time, so building one myself was the obvious choice, and I know a lot of other people are in a similar boat. The main construction is 3/4 plywood, construction screws, staples and marine grade carpet. I designed it to be able to fold up so my rear seats could remain in the truck and to have drawers for storing camping gear.
The setup has worked well for us for years and hopefully it can inspire you to get out and build your own!