I have good luck with 1/8 in .mahogany plywood. a 4x8 sheet is about $11 . Washington is known for rain and dampness, I have door and trim panels over 10 years old that have never warped and still look as good as the day they were made. .abs panels cost me $17 for a 3x4 sheet. I think the plywood is easier to work with. the only drawback....... slivers,

I use ABS for every panel I have built in the last 15 years. I'm in Illinois so we get hot and humid in the summer and below 0 in winter. I have not had any problems with ABS. Now I cant say how it would perform in hot dry climate like Arizona.

Direct heat will make ABS expand and uncovered ABS in the sun will swell. I would suggest at least 1/8" closed cell on top before you cover. I do think that a package tray warped with thin black vinyl would swell in the sun. I have never had a problem with it just adding the 1/8" closed cell.

The versatility of the ABS is hard to beat. It is also very easy to work with. Score and snap or use hand or electric tin snips to cut. I'ts easy to drill, will hold staples well, heat and shape (thermoforming). If you bend to far like 45° without heat it will snap. So just use a propane torch to slightly heat up the bend area. Around here a 4X8 sheet of .090 is about $35. If you mess around with it enough you can make some pretty cool stuff

That's awesome Andy! I can't believe you were able to bend and shape the plastic like that. Very cool

that´s a cool idea....was it vacuum formed and heated?..to me looks like that...so in that case build a wooden sample and vacuum form it over....

Yes heat then vacuum. I just built a vacuum box out of wood and used pegboard for the top then a shop vac inside with the switch wired through the box, along with exhaust port. The vac could easily be outside, also wouldn't need to wire in a separate switch. Eventually I plan on building a heat box/oven to be able to control temp of the ABS, but for now I just use my acetylene torch, or a big propane weed burning torch to heat the ABS. Its real hard to control the heat that way though.
And yes a wood sample to form over, or carve styrofoam wrap it with plastic wrap and a layer of fiberglass over that. At that point you could just make it with the fiberglass but that is a lot of mess and expense in matting and resin. In either case the heat of the thermoforming or resin chemicals will melt the styrofoam. Wood is a better model but foam is much easier to carve.