Ouch! Rather harsh treatment of a soon-to-be-collectible dust cover...
How about PRINTING a full-size copy of the cover, and mounting THAT in your rear or side window?
That works. Of course, at the friendly local Kinkos you could do that for a buck or two using color printer, and of course you could increase it to about 200% in size, to REALLY get the message out. The idea is to be quick, to the point, cause the person to ponder and figure out for himself/herself what it was he saw, and maybe also cause some to go buy the book themself.