I made one by carefully laying out the circumference of the hole on a piece of steel, along with the locations of the grooves. Drilled holes through where the grooves are. Drilled out and bored to size to fit a barrel nut. Place over the barrel nut, tap brass pins into the "half holes" which remain, and the nut grooves.
Some Savage nuts have round bottom grooves, others square.
Another way is to place small rods in opposite grooves, and slip a large wrench over. The rods will prevent the wrench from turning, and will allow pressure to be applied to the nut.
By far the easiest way is to buy a wrench.