Cut it the best you can - use a sander to correct large cutting errors and a sanding block to do the final squaring of the cut.
Fill the butt stock with rigid, OPEN-cell foam and let cure. Hollow out the foam so that you have a 1/4-3/8 inch recess in the butt stock. Run a fence of masking tape completely around the buttstock base. Use an epoxy that has decent flow characteristics or fiberglass resin and fill the the recess to slight overflowing above the cut and let cure overnight.
Use the sanding block with 80-110 grit to level the buttstock base back to the original cut level, finishing with finer grit such as 220-320.
Now you have a solid base to connect the recoil pad with screws or with screw-pillars. I can go into more details such as alternatives to foam but best if you pm me.
