I have seen several British Double rifles that have had their stocks "steteched".

Using a matching wood spacer looks pretty good. The best work I have seen actually had a distinct small grooce hwere the 2 wood peices came together. I think this looked the best.

I have also seen the wood spacer and recoil pad covered with pigskin, making it look like it has a 2" recoil pad.

Also you could get a quality [like a Galco] leather slip on or lace on recoil pad.