The stuff I used to lengthen the stock was a material that appears to be the same as the butt plate. I got it from a local guy who does put on recoil pads. It is two layers of this material that is about 1/4" thick. So that made a total of 1/2", then of the course the original style butt plate.

I am looking forward to seeing the pictures of your gun when you can get them.