Here are the measurements for using screws to attach the POD Mount. These are sized for the next-length size appropriate screw and using the rubber feet as well for cushion bumpers.
The actual well depth in the metal POD shells across all models is .40" or 10.2mm.
If theyou wish not to use the bumpers, you should use either washers or nylon standoffs on the following mounting screws to make up the space difference. A round-down size screw would be too much to use and the threads may not bite into the cast metal shell correctly.
POD/ POD 2.0/ Bass POD
6/32 machine
1.0" or 25mm
1.25" or 32mm
POD XT/ Bass POD XT
6/32 machine
0.75" or 19mm
1.00" or 25mm
FBV POD POD 2.0
6/32 machine
1.0" or 25mm
1.25" or 32mm