I've used two DIY. Both same basic idea. Here's the latest:
Has a pull out shelf on either side.
The l-track is easy to use to tie down fridge, battery, boxes with gear, etc. Plus it's level with the back, so sleeping.
The first one I covered with LineX which was a mistake as even 6 months in, it stank a bit. So I went with some polyurethane on the second one and just cut an exercise mat to fit that I leave in there when I'm not off road.
It uses the 4 factory tie down spots. Completely reversible.
I got the idea from others on the forum ... so it's a tried-and-true practice.
It is 3/4 Baltic birch.