I said that one goal of this was to keep things from sliding around.  I probably wasn't exactly clear now that I have spent some time reading old threads.  The stall 3/4" stall mat does not move inside the bed.  Heck, my wife and I struggle to get it in and shifted to the right spot without any extra weight!  The camper has a fiberglass bottom like a boat hull.  There is an access panel under it (not sure what it's for), but I couldn't say how much that helps keep things in place.
I think the reason the camper slides more than anything is that I don't have any forward blocking (will be addressed) and the fastguns have a spring loading which I might not have secured sufficiently (will be addressed).  The other issue is raising the camper slightly.  I would like to increase the height by 1/2" as well as provide some support aft of the bed which the stall mat can't do.  I could just get 1/2" plywood and cut/glue it together as desired and then just use the 3/4" stall mat like I've been doing.  The composition is identical to the 1/4" and 3/8" material that I'd be replacing it with.  I'm sure I could source some scrap 3/4" material to make that work.
The only reason I have not considered that to this point is that NL has specifically said they don't recommend 3/4" stall mats because they believe they are too hard.  I'm not clear how 3/8" or 1/4" stall mats with 1" plywood would be less hard though.