What your looking for doesn't exist, so you have 2 custom options.
1) custom built device, might work, might not, but certainly will cost $250-500
2) cut and re-thread barrel to 18.6" and 1/2x28 and use standard muzzle device options.
Personally I would go the latter route as I am not worried about maintaining the OEM form of the rifle, rather I prefer improving designs.