Well here is my Jim Williams mount. It is a little different that I thought. What I will probably do is sandwich each horizontal slot between 2
L- brackets. Then the L-brackets will be bolted to the chassis. This way I can move the mount for pinion changes with out cutting a big hole in the chassis.

I though the mount mounting screw would thread up into the bottom of the plate...by bottom I mean the surface that sits on the chassis.
I was probably going to use some stick on velcro pieces and velcro cinch straps.