Up until a month ago, I worked in a collision repair shop and I have first-hand experience with automotive adhesives.
A few years back, the hot setup was to glue, instead of weld, panels onto wrecks.
I've pulled a few of those repairs apart.
The panels come off with your bare hands. No chisels, no hammers, not even a pair of gloves. Just grab the panel and roll it back. The glued joint pops apart and all you have to do is scrape off the adhesive residue.
As for acrylic mirrors, they won't stand up to the elements for much more than a year. They'll develop fine cracks and will discolor.
If you want to use an acrylic panel, use windshield urethane glue. You have to heat it (glass companies have special ovens that the tubes fit into) to get to even come out of the nozzle.
It MUST be heated prior to application, otherwise, it won't adhere to the two surfaces.
I suppose it'd work with a stainless firewall, too, but you'd have to clamp it well before it dried or the panel would stick out in spots.