I'd buy another stock (yes, it's not within budget) before i started cutting up that Israeli unit.. OR go to one of the Sig forums to see if you can do a trade or barter.

BUT... if you really wanted to do it. I'd go with a secured internal rail with nuts tig'd on it at specific intervals. Run 2 - 3 holes in the stock for attachment. You'd have to inlet the stock for the rail, so it sits flush . Then glass it over.