If you have access to a good stand-alone belt sander that is the best tool. You can concentrate on working with the stock and don't have to worry about controlling the tool. A sanding drum on a drill will work, if care is taken. I think the sanding disc is not a good choice, though, and harder to control. (If using a belt sander, make sure you have a good cloth belt, and keep the tension right. You can't have the belt curling up at the edges because it will then dig into the stock where you don't want it.)
Smokem has the right idea, but I think you will find masking tape easier to work with than duct tape, plus it comes off easier and leaves less residue. You can dig into the masking tape as long as you don't dig THROUGH it.
Just make sure to extend the lines of the stock in the pad. Nothing screams "amateur bubba" like a recoil pad with the bottom cut parallel to the bore.