Thanks, Harry.
Here's the pan/enclosure before it was mounted in the trailer.
I tried to re-create the support the AC would have if it were mounted in a window. The weight of the unit sits on the inside edge of the pan and the face frame of the cabinet. (the windowsill) It also sits on two raised, PVC ribs that run perpendicular to that in the bottom of the pan that I hoped would absorb road shock. They're tapered towards the outside for proper drainage. There is also a 2x4 mounted across the top, behind the flange on the AC that it is screwed to. (the window sash)
You can see it in this next picture, before the finished cover frame is installed.
I bought my 1/2" board at Home Depot. There are two part glues that are designed for it. (Double barreled calk gun and mixing tips.) I got mine through my employer. If the Depot carries it, I missed it, but you should be able to search online for it.
Note: Cellular PVC is not the same as Starboard. It glues much better and is lighter in weight.
-------------------
After a trip around the neighborhood, I think I'm going to be pleased with my WD hitch. After much fiddling and adjustment, I achieved zero front end rise on my truck, and about an inch of squat in the rear. The ride and steering are solid. Much better than with the rear air bags I've been using.
I had to order a different shank to get the drop I needed. I've since cut the extra off the bottom for ground clearance... That's solid 2x2 steel.
I also had to clip a corner off of the chain hangers to get them back far enough, then drill holes through the diamond plate for the mounting bolts.
Nothing comes easy.