You'll need to find this horizontal square aluminium tubing just below the wood piece (in the photo below) behind your interior side panels. If the top of your interior side panel has screws then undo a bunch of them and take a peek to see how far it is down from the top of the interior side panel. Otherwise figure out another way to find that square aluminium tube, maybe an ice pick or other very small poke device that you can probe right through the interior side panel. I guess you could even cut away some of the interior side panel that will eventually be hidden by the hardwood slider strip.
The hardwood slider strip (any hardwood will do, pick what you like, shown is 1x4 oak) is screwed into that square aluminium piece so that the top of the hardwood will be flush and level with the top of the cab over part. I used a spacing from screw to screw of about 6", however, 10" apart is what I've seen from the factory.
If done correctly, the slider plywood piece (I used 1/2" thick plywood with an oak veneer, cut to the size you need) will smoothly go right across from the cab over to the hardwood slider strip. By the way, the photo below is showing the overlap of the slider plywood piece with the cab over section, about 3" or so.
You'll need some sort of a STOP device to keep the slider plywood piece from pulling out too far.
After cutting the plywood slider piece to the size you need, you'll need to span from right to left across the rear part of the plywood slider piece for stiffness using another piece of hardwood. I did a rabbit or dado into the hardwood like shown below. The top is to the left (adjust the height for the thickness of your bed cushions) and the bottom is to the right. I just glued and screwed the hardwood onto the slider plywood piece.
Remember to make the end notch as shown above and below (in place and pulled all the way out).
