This is easy to do. Depending on what you have to locate it in. We have our own maintenance units at each location, except for our one at our office.
We put a mail slot on the outside of a building where our stuff is located. You will need to cut a hole for it. Then use some heating duct or some other kid of metal shoot for the payments to go through into a milk crate.

If you need a better image I can take some pictures.