Cut them to the desired length after taping the area to be cut with masking tape. I always used a mitre saw or table saw. Make sure the cut is square, block or disk sand as needed to get it square. Rough up the front side of your new Decelerator pad, don't kid yourself they are the best and easiest to work with. Glue pad in place with Loctite 416, fast setting superglue type gel. When glue is dried tape the stock off and grind the pad using a disk or belt sander. I know that the thin layer of fiber glass doesn't seem like much to glue to but I've done hundreds of glass stocks this way and never had one come off. Don't cheap out on the glue or use cheap 5minute epoxy, they will fall off then seen it happen more than once.
Got any pictures of what one looks like cut? I thought about cutting one down for my kids. My thought was to glue a smaller base to the recoil pad that would perfectly fit inside the hollow stock giving a larger gluing surface. I was concerned though because I didn't know what one looked like inside. Are they hollow inside enough that my plan would work?