I would suggest taking a look at the KelTec KSG.
Holds 14 rounds.

I have one shotgun with a side-saddle for extra shells along with a sling that holds extra rounds.

I have another shotgun in a carrier sling I can carry on me. That carry sling has a shotgun shell ammo pouch attached. Here are iPad pictures of the shotgun carrier with the pouch. The pouch holds 25 shells. The pictures are not the best as I just took them with my iPad in the closet.

Ugh. I need to find a new photo hosting option as all the ones I have used have discontinued.

This message has been edited. Last edited by: StorminNormin,

 
Though designed for competition, the Calfornia speed stripper belt carrier would work fine for everyday carry, and fast reloading. Click the image for link to the product at Brownells.
 

My 870 has a SpeedFeed stock that holds 4 spare rounds and a 6 round mag tube, so 11 rounds. I'm thinking about adding a sidesaddle as well. All 00 buckshot. Not as maneuverable as a handgun, but with the nervous tremor I have I can't rely on being able to hit anything with a handgun. I picked up a 20ga Shockwave on sale a while back but haven't had a chance to really practice with it so I don't know if it would be suitable for HD or not. A sidesaddle would probably be the solution for extra ammo for that gun too (assuming they even make a 20ga sidesaddle).

I have a 6 shell side saddle. I also have a sleeve on the stock that holds 5, although most times it is not on the shotgun.

I have two 25 round slings hanging in the closet, for just in case, that I can grab quickly.

But, here is a video that you might like. It is from nutnfancy guy on youtube about shotgun shell carriers.

https://www.youtube.com/watch?v=wnLbe5q3-Lo

This is my closet HD shotgun. It holds 8 rounds. I can't imagine I'll need more than that but just in case.