I use 4s 5000, easy digging is over an hour, hard digging about 40-50 minutes.
The latest models from Fumotec have rubber feet about 8mm tall. If nothing else shorter available, I'm sure you could cut out a square piece of rubber from tractor tire tube, the type that are filled with chloride as they much thicker than a car tire tube. But maybe a car tire tube would be enough.
