Make some sandbags out of the cutoff legs of old canvas or denim pants. Fill them with sand, and use those big zip ties to close off the ends. Make several; at least two for the gun, maybe one to put under your wrists and one under your elbows, depending on the height of the bench. You want the front lower corner of the frame and frame barrel intersection jammed firmly into the bag. Try to not have any other part of the barrel supported by the bag, all of the support for the gun should be on the front and lower part of the frame (basically, everything between the basepin and the front of the trigger guard. Some people rest the butt of the gun on a bag. That's fine, but keep in mind that will probably affect your POI when you shoot from a position without the butt of the gun supported.
Grip is critical when shooting for small groups, and trying to zero sights. It needs to be the same every shot. This can sometimes be a problem shooting off a rest, because you're not supporting the weight of the gun with your grip, and the pressure and position of your hands can vary shot to shot. One thing I like to do is establish my grip with the gun unrested, just a few inches above the bags, so I'm holding the entire weight of the gun, then lower it down and press it into the bags, being careful not to change my grip pressure. Do this with every shot, so it is the same. Vertical stringing on the target is a good indicator of changing grip pressure on the gun from shot to shot.