I have an over the dash mount so it's different than yours but maybe there is something useful to you in my pictures. See HERE
I made two sides, top, back and bottom out of 3/4 inch plywood.
Probably overkill but I wanted 3/4 on the right side where I installed the articulating arm for strength and I just wanted to buy one sheet of plywood.
The arm is held with two 1/4 inch bolts with fender washers under the nuts. The top one goes through the steel framework Winnebago uses around the cabinet.
I put the back piece of plywood in last so it could be easily removed to have access to the area behind the cabinet to run wires to the electronics in the cabinets on each side. (5 disc DVD/CD/Mp3 player, receiver amplifier, satellite receiver, Wilson cellphone amplifier, Wilson Trucker cellphone antenna downlead coax, and and Verizon aircard).
Before installing the plywood pieces I covered them with laminate flooring left over from my flooring project. A couple of the pictures show what I used for the travel hold downs. I put some weather stripping on the back of each one to protect the TV face.