I have shortened a couple of synthetic stocks . In most cases I found that they had plained on someone shorting their stocks and had left enough of a ridge in the top and bottom that you could still drill and place the butt pad screws back in .
If not you can either make a wood filler and glass it in or fill the stock with steel wool or something like that ,leaving a space of about 2 inches from the butt and fill that space with Accra-Glass or some other kind of fiber glass . That will give you something to screw the butt pad on to .
