I agree with Fender3x about not using filler. Use some hardwood dowel, cut it roughly to size first, then glue it in place. I did something similar on my EX-1 kit where I've routed the body for the selector switch in the standard location on the lower horn, filled the large hole for the old selector switch with dowel, then cut the excess height down with a chisel and then sanding, then redrilled it to fit a standard potentiometer.

Epoxy filler won't have the compressibility required to hold the inserts firmly. Try and force an insert in to a hole drilled in epoxy filler and it will probably crack.