M870 Shotgun rear sling mount

[Bulabulapesci]

I recently saw a pic of what looked like a TM M870 Shotgun with a magpul rear sling mount fitted between the stock and the receiver. Does anyone know if this is do-able without horrible modding? If not, does anyone know of a sling mount that might work - I'd like the slung shotty to hang in a good ready position. Any help or advice would be greatly appreciated.