Easy but not cheap, the Moen 1920 remodel plate:
»www.amazon.com/Moen-1920-Remodel ··· 013C3VI2
Of course, this works best if you're replacing the 3-valve system with a Moen shower valve, which is what I'd highly recommend. If you can find an old Moen Chateau valve, buy it and use it. By-far the most trouble-free plumbing part I've ever used. I've gotten ~25 years between cartridge changes on these in areas with absolutely nasty water that clogged everything else. Avoid their Posi-temp line if you have hard water and won't be using the shower all that often as these tend to clog up with sediment and you'll have a rather cool shower. Moen will gladly send you free replacement cartridges for the life of the house though, so if you don't mind changing cartridges every few years, the Posi-temp stuff isn't too bad. Pulling a cartridge isn't that much of an ordeal; I just prefer to have things be as near maintenance-free as possible.