I don't have much luck getting kit components to fit what they are supposed to fit and having seen the results of unskilled labor cutting holes for things, I generally roll my own..
I used a material called King Starboard for the mounting plate. It's a hi-tech plastic material that's used in the marine industry and is completely UV resist.
I cut the square large enough to completely cover the existing cutout and screw holes. Since the bottom half of the cabinet was already unusable because of wiring, I put the PI EMS system in the lower half of the cabinet and mounted the display behind the door also and just used the existing door. A bead of white silocon sealed the Starboard.
Shown below, I used the same method on a previous trailer so it works on either fiberglass or aluminum sides. On this one, I used butyl putty because it looked like that is what was udes on all of the other outside openings, but white silicon would have been a much nicer installation. Maybe I should market this as a kit. It sure beats trying to fill gaps and hit screw holes and it completely hides all of the ills of the existing installation.