Remixed from commtechva's fantastic design: https://www.thingiverse.com/thing:2749898

"12 Squared Shell Holder"... 12, 12-Gauge Shotgun Shells.

My wife and I each have a Remington 870 with extended tubular magazines, so I needed an extra shell in my holder, I also wanted to be able to "Hand Off" a block of shells to my wife for her 870 if needed.
There are matching holes through the front and rear "12 Squared Shell Holders" for a 3mm bolt to hold the two halves together as one block, or you can use a PLA Pressure-Pin to hold them together while still being able to separate the two shell holders quickly if you need to hand off a block of shells.

Bolted together, or separate, they fit very well in a standard "M4 Mag Pouch" in a Gear Vest.