Consider a Grip Pod. I use
the Grip Pod LE on both my M&P 15-22 and my Rock River - Elite Operator. The Grip Pod is very well made USA product. Works great on a bench or laying pone. Two versions the LE has a heavy duty polymer legs and the military version has steel legs. Consider the LE model unless you plan on doing a lot of drop, flop, & shoot action carrying 60lbs of gear.
OBTW... consider adding a section of black butyl bicycle inner tube (26 x 1.9 - 2.2) to the handle. After you fit the tube band to the grip, use a rag and a little acetone to make the band slightly tacky.