If you want to do this, get some 1" tube webbing from a rock climbing shop. It is soft, flexible, and has a slick surface.
Tie the ends together to make an endless loop.
Pass the loop *behind the buttplate*. Drape the rest of the sling over the front of the bench and attach the weight. You could probably pass the sling over the forearm rest.
This way the counterforce is applied to the butt , as the stock was designed for. The sling and weight act on the stock as though a larger body is behind the rifle, instead of pulling forward on the forearm and making the rifle the object of a tug-of-war.