Grind to fit advise
I have installed many of these pads. Do not try to sand it while installed. Install the pad, scribe a line on the pad base around the stock, remove the pad and sand to the line. Sand away from the plastic base, do not have it on the bottom or your finish will be very rough and your sander may try to grab it. If you are using wet/dry sanding belts then try to lubricate it with WD-40 while sanding. You can finish off the pad with 0000 steel wool. It will be difficult to do a good job without a holding jig but it can be done. Good luck.