important to note you can only do this mod on the XV sized cans, the smaller can models would require the proper fox reducing kit.
i did it on an RFX i had, made an awesome difference, but swapped the whole can out for an LV one. which was even better.
about to do the same to my new bike (the shim mod) as i have to run it "Just" too firm to stop bottoming.
if memory serves, the shim i made was around 22mm wide, altering the width adjusts the volume reduction, but you need to make sure you don't cover the holes in the inner can.
can't recommend doing it enough