This is the measurement of the cleat opening:
Box width 5/16"
Box height 9/16"
Slot width 3/32"
Slot height 7/16"
Total length 1"
The thickness of the Lexan is a perfect fit! Not sure what the thickness is though...
I think I'm gonna make the cleat mount 1" wide by 1 3/4" tall. Same fer the Tetrax Fix shims. Should be able to keep the "thickness" of the touchpad to just one 3"x2" sheet and the rest will be the shims (2 fer the Tetrax and 1 fer the mount) and the mount. This comes out to about 9/16" thick. Not too shabby.
I should be able to bond the Lexan together with some Methyl Ethyl Ketone, IPS Weld-on #4, or PVC cement. I think I will stick with the PVC cement as I already have that on hand.