IF you are using one of the plain jane stocks, and don't mind possibly ruining it. I took one of my old stocks, took a small block of wood. Shaped the bottom of the block to match the top of the stock. Once it is a close fit, glue the block to the stock. Once it is completely dry, use wood filler and finish shaping it. Once you are happy with the shape, let it set and dry completely. Then seal it. The only problem is you really have to paint it.
I think CW has one, he mated a block to and put a clear finish on. But his fit a lot better than mine. Plus mine was On a 22 Marlin. I lengthened and the color would have been funky. Will see if I can find the picture.
Lonny