Alright, that's not really a canopy, more like a shield or body armor.
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It wouldn't be hard to make some thick foam mats with straps sewn to them that can be rolled out and fastened to cars in under a minute each. Maybe toss in a couple plastic strips to make it unroll itself like the reverse of a slap koozie. Weather forecasting is pretty good. It's not like you don't have decent warning. You can cover everything out back and leave the cars that are on display out front uncovered until the last minute.
Tossing a mat over a high roof van or large pickup wouldn't be easy but the hordes of crossovers could be well protected.
OK, maybe it does almost sound feasible, but I wonder what someone with outdoor car dealership experience would say? Is this a practical idea? How often do you need to unroll those mats?
Maybe a few be stored, rolled up, under vehicles at the back of the lot that are less likely to be moved?
And the rest could be stored indoors. They shouldn't be heavy, so maybe they can be stashed in the ceiling, or somewhere high up out of sight?
You could even offer a deluxe version with a CO2 cartridge.
You made a good point, anyway. Foam could absorb water, and air, and get dirty, and smell bad.
I was musing at the time why someone on Amazon wasn't selling some kind of hail protection padded car cover.
I have tons of great ideas, but I never connect on them. Then I watch other people make a fortune off that same idea. Shrug...