Grab..........I have built a lot of barn guns.....no buttplate. I saw the stock off with a slight curve, just like a lot of buttplates, then round if off to resemble a buttplate. When it comes time to finish it, do the same thing that you are doing to the whole stock. You will find that a sealer will really soak into the butt, you may want to add additional coats
in this area...................Don