but I don't own anything in them!

A "cooler-dor" is just that. A cooler (ya know like an Igloo brand cooler. one that you'd take camping?) converted into a humidor.

They are inexpensive compared to the cost of a good humidor especially when you consider how large you can get them.

You will need to provide humidification for it. That can be done in many ways and you'll need to have some empty boxes for singles.