Sweet build! I have taken inspiration from your build and am designing a similar setup for the back of my Tacoma (3rd Gen short bed). I will put together a more complete description once I build it, but here is the design, in case others need some inspiration.
I liked the sliding "stretcher" design for vertical storage, but wanted to be able to have a flat top. So, I am going to use some 80/20 aluminum sections to serve as cantilevers for when the slider is in the lower position and as supports to put the slider level with the rest of the build. It will also allow me to put some tie-downs in there if I need them.
I made a few other changes, like not making the drawer the full length, adding some cutouts on the drawer to access my stove's knobs, and a little "tube shelf" on the side to hold small things. I also will make a little extender piece to be able to drop the tailgate and extend the stretcher out a bit more to sleep full length on clear summer days.
The psych is high for getting started into getting wood and building! Any tips and thoughts are certainly welcome!
Other inspiration:
https://expeditionportal.com/forum/threads/i-made-some-drawers.189395/#post-2425844
https://expeditionportal.com/forum/threads/latest-project-truck-drawers-sleeping-platform.47516/