I am tall and have long arms and a long neck. I often need a length of pull that is about 1" longer than comes on most factory stocks. As a quick, cheap, easily reversible modification on a rifle or a shotgun, I often use a slip on recoil pad fitted over a screwed on recoil pad.
It works best if the rear surface of the permanently mounted recoil pad is perfectly flat, not curved. You can sometimes sand it flat if necessary, but you will probably ruin the pad for use without an additional covering. Then you can use a piece of double sided carpet tape between the two pads. I have not had any of the problems with the arrangement which have sometimes been reported by other shooters.
On heavy recoiling shotguns and rifles, I have used a slip-on Limbsaver over a screwed on Limbsaver or Decelerator pad. You're looking at over $80 worth of recoil pads on one stock, but it fits well and makes 3" 12 ga, .300 win mag, and 338 win mag pretty comfortable, even when firing at the bench.
Oneounceload's recommendations are also very good. When using two pads, there is the possibility of slippage and wobbling, so his recommendations would probably be best for long term use, good appearance, and a semi-permanent installation.