Remington 870 Mcs (modular Combat Shotgun) In 18-barrel Patrol Shotgun Configuration, With Tactical Buttstock Where Can I Find A Shot Shell Carrier That Will Fit My Remington 870 Factory Top Folding Stock?

Where can I find a shot shell carrier that will fit my Remington 870 factory top folding stock? - remington 870 mcs (modular combat shotgun) in 18-barrel patrol shotgun configuration, with tactical buttstock

I have a Remington 870 with a factory top folding stock my question is where can I find a shot, the shell can be placed above the factory folding stock on. I saw him throw ATI Universal shell company that has their own version of wrinkles, work in mines or do I need to drill holes in my stock from the factory?