I have had about 7 or 8 window units A/C's stolen from me over the last year or so. It happens right as the tenant is moving out. Sometimes, in the case of an eviction, I think the tenant may have stolen them, but mostly I think it's just opportunistic scavengers who see the U-Haul in the driveway and come back that night to take them. Several of our units are in low rent areas, where central a/c doesn't make sense.

How is the best way to secure these things so they are less likely to get stolen? The only thing that I know worked once was star head screws, accidentally. They came and ripped out a big piece of drywall trying to get the A/C out of the wall, but I guess all they brought was a phillips screwdriver. 

I was thinking of installing some plywood pieces for each side of the A/C, maybe that would make it look too difficult to pull out?