you could take a piece of walnut, beech, birch, poplar, anything that might make a gunstock, cut it to the thickness you need and trace your buttstock on it, saw out that shape to about 1/8 oversize and then predrill it with a 1/8" drill, place against your existing buttstock and complete the holes being careful to leave about 1/8" all around. Take into account that your stock will continue to get larger (taper) as it goes towards your buttplate/recoil pad. Install one screw and carefully drill a 1/4" hole in the other predrilled hole through your spacer and about 1" into the buttstock. Place a 1/4" hardwood dowel in that hole and then remove the first screw and drill the other hole out to 1/4" the same way. After being sure your existing buttstock and homemade spacer are competely flat and level so there is a good tight match where they join cover the dowels and each piece of wood that will be mated together with a good waterproof exterior glue and join together with the hardwood dowels firmly pounded into the holes and the 2 pieces of wood clamped firmly together. Let dry and refinish just as you would any other piece of wood or gunstock. The different woods will take stain differently but when completed all you should see is a fine line where the stain is different. The stock will as strong or stonger at the joint than a single piece of wood.
I always advocate that BEFORE shortening a stock for anyone that you predrill two 1/4" holes in the back of the stock where you have removed the buttplate as deep as the piece you are going to remove plus 1", save the piece you remove and you can later replace the piece you have removed with all the grain and edges perfectly matching by simply glueing it back in place with 1/4" hardwood dowels. Also use as thin a kerf blade as you can that will cut perfectly straight and flat with little splintering. Before cutting wrap the stock with painters or masking tape to reduce splintering, and let the blade do the work, go slow and don't force it through the wood... This is much easier than trying to match it exactly years later. Good luck and have fun with it, that's what these guns are for....<><.....