I am surprised that a "master gunsmith" could not figure out how to resolve your problem.
It is fairly common now days to find polymer stocks that are hollow. The recoil pad, if screwed in, usually engages a couple of areas on the stock that are cast into the stock during production. And yes, when you shorten the stock, these will often be cut away.
I've seen a number of stocks of this type that were shortened (cut) and then either a wooden block or epoxy plug crafted and fit into the cut end of the stock. One must usually cut a hole in them to allow access to the screw/nut that holds the stock onto the receiver. Once in place, screw holes for attaching the recoil pad are drilled. Yes...the recoil pad is then cut to fit, finished, and things are ready to go. A new recoil made by needed for fitting. Over all, a small price to pay for obtaining proper fitting to the firearm.
Edit: You have a nice rifle there. I hope you can get stock set up the way you wish. Here is my 10 FCP HS-Precision.