I just finished a 300WM build using a MagPul Pro Chassis. I am pleased with the chassis overall but wanted some extra height and contact area at the rear bag. I came up with a solution that is very economical and easy to fabricate. It works very well both off the bench and prone-supported utilizing the Triad Tactical Pillow rear bag. You will need a MagPul 7-slot MLOK rail section with T-nuts, a 5.5 inch KAC rail cover, and 2 10-24x3/4 countersunk machine screws (available at Home Depot or Lowe's, etc.). It does not interfere with the folding function of the chassis and I mounted the rail section where the vertical adjustment of the recoil pad is still usable with chassis adjusted to minimum LOP. 
