The butt cuff should do you well.
I have a 6-shot Tacstar sidesaddle on my Mossy which is alright. It hold the shells okay, but once you get ta' shootin' the shells can shift up and down a bit.
Activ was right on with the Mesa tactical sidesaddle. I just bought one of those for my incoming Rem Tac-2 and it blows my Tacstar saddle away. Theres a rubber insert in the Mesa saddle that should hold the shells much better than the Tacstar. The Mesa looks much more durable too, but for double the cost it better be.
Having the saddle attached might bother you at first, but I hardly notice mine anymore.
Theres also the Speedfeed stocks which can hold four in the buttstock. Even know the Rem Tac-2 comes with a pistol gripped Speedfeed I bought the pistol gripped III model that holds four shells.
Other than those, you can find bandalier(spelling
) slings and little clips that hold 5+ shells with a belt clip attached.
As far as the dozen round shotty goes...
My Mossy holds 5+1 plus 6 in the saddle for a total of 12.
My in transit Rem Tac-2 will hold 7+1 plus 6 in the saddle plus 4 in the buttstock for a total of 18
The Mesa saddle also comes in a 8 shot version as well, but a forend that doesn't reflex back over the receiver is needed.