On my XL350:
I also used the blue sleeping bag ground pad... mine had the "bumps" so I turned one piece over and fit into the other.... Got maybe 1 1/2" height max out of that. It was good but pretty firm and hard enough to be difficult to upholster over. So I used a piece of "deluxe" carpet pad... 1/2" thick, softer, and I stapled that to one side of my plywood cut-out. (seat base) Pulled it tight up, over, across and down and stapled it on the other side. Did a similar thing with the small riser at the rear.
(haven't done a tail section yet....)