How about a textured ABS panel like mine?
My dash kit had a little bow in it, but not quite as bad as yours does.
I can make pretty much any size - top or bottom mount. I used some heavy duty double sided 3m or Scotch brand exterior foam tape for mine (same tape I use for my Ford ovals) and it sticks REALLY well (just clean everything with alcohol before installing).
It's about 1/8" thick so it's quite sturdy yet it's not too bad to cut if you want to add switches, voltmeter, temp gauge, etc.
Drop me a PM if you'd like one.
BTW, I almost got that head unit too