If these people rent the property a guard shack is going to be almost useless..... 5 minutes and a crow bar and bye bye transformer. Those things are good for public places where a little bit of security will stop most people walking by from messing with an irrigation timer or stealing a backflow device. But if its at the house you rent whos going to watch you and call the cops?
If your truly worried about theft and they cant be installed in a locked utility closet then just install underground transformer. 99.5 percent of renters will never know that there is a transformer under some mulch.