I actually did this a few years ago having to replace my wood with synthetic. Normally where the location of the stock stud would go the plastic is a little thicker. Get your stud set and measure up from the butt of the stock where you want the stud. Be carefull to choose a drill bit just undersized of the stud threads to give it some bite when you screw it in. If you want after drilling that hole you can use a larger bit to somewhat counter sink it just a little for apperance but it's not nessesary. You have to be very carefull doing this cause my bit I used to countersink my stud on the stock grabbed and I almost drilled too far. If your handy at all this shouldn't be a large task, it's no different than drilling anything else for a screw. Just go slow, the plastic will grab the bit and before you know it you've gone too deep. Sorry if I made you nervous but it can be done if you have a gunsmith near if your unsure doing it yourself. As far as the cap, you can pickup a replacement for the 870 withthe studalready installed. Good luck