Thanks to Missing Link, who gave me some ideas about what to do. His post is on page 2 of this thread.
I pulled the rear seats of our 2019 Tundra doublecab. It was a straightforward task to locate a 2 foot wide five foot long, 1" thick, board at Lowe's. 8" risers along the front and back, with a center brace running fore & aft. The trick with the center brace, to avoid the center hump in the truck floor is to offset it, so it is two feet in from the driver's side and three feet in from the passenger side. A little work with a multi-tool in the center of the front riser to straddle the center hump in the floor. Also, to level the platform, I attached a couple of 1" thick pieces of scrap stair tread under the front riser, one behind each of the front seats. That's it.
We can still reach under the front seats. There are lots of attachment points for straps and such on the rear wall of the cab. We put a camp table, and stove under the 3' deep side of the platform. And a couple of other longish tools. In the 2' deep side we put a couple of camp chairs and a slim tool box case. Behind the platform is room for an air compressor, a larger tool bag and a bunch of other stuff. Tuffboxes and a cooler fit on the platform very well.
Seems to work well. In a couple of days will take it on a long maiden voyage. Will report back with any problems, needed changes, etc.
The project took about 4 hours total, including going to Lowe's to find the wood.