The current potentiometers that we are using in the pedalSHIELD kits are the Alps RK09L114001T (
mouser link
)
They have a shaft of 20mm which is just perfect for the cover, I remember that there is another model with a shaft of 25mm which also fits but it is a "little" big for the job.
In the image below you can see how all fits together:
Hi conaty,
There is another possibility for you.
The result is a faceplate a little bit inclinated to screw the footswitch and potentiometers and switchs.
Just cut the round top of yours jack sockets to reduce the high and make two "dremel" windows on the faceplate near the entrance of the jack socket.
The faceplate is now less sexy but it permits to screw all the parts.