For unit-by-unit inspections how do you hold all your unit keys?

I work at a property of about 100 units and a lot of these folks aren't home during the day when inspections are taking place. How do you hold the keys? I'm currently using a cardboard box but it can only hold about 30 keys before I need to go back to the office and switch them out. I'm just trying to save myself and our vendors time by eliminating that step.