Question: How to test the fit of your beer barrel?
Option 0: Cut the slots for the garbage can handle deep enough to allow the barrel to hang with the rim at a different height as the rim of the metal drum.  Test the fit of the liner and make any changes necessary. File any sharp rasps off of the metal barrel.
Option 1: Cut the slots for the garbage can handle deep enough to allow the barrel to hang with the rim at the same height as the rim of the metal drum.  Test the fit of the liner and make any changes necessary. File any sharp rasps off of the metal barrel.
Answer: 1