Modify the stock that comes with your Rogue One Captain Andor Deluxe Blaster (aka the NERF Raider Stock) into something a little more screen accurate. This is designed to work with the stock extended into the middle position. Once adhered to the piece, the position will be fixed. Use a putty or other similar substance to fill in any seam lines. Light sanding may be required.
My thanks to YouTube user J0D0CAST whose designs were used as the inspiration for these parts.