Ciampino Parking Lot

41°48.6'N 012°34.9'E
To get to the lot, continue past the Ciampino airport big roundabout, continuing no Via di Ciampino until it curves around.
There are two lots: you want the one on the right side when you turn in at the "Cheap Parking" sign.
Show your military ID and you can park for €4.50/day.
They shuttle you out there and the reciept they give you has their number.
Call while the plane is landing and they will be waiting outside by the time you get to the airport.