Good evening.
My advice would be to try a Pachmayr decelerator slip on pad. I need a stock extension on most shotguns, and the decelerator pad extends the stock by just under an inch which suits me fine in most cases. It also absorbs quite a lot of the recoil which can't be a bad thing.
I've never tried a convex pad, and can't see the point to be honest.
Cheers.
Stewart.