I put this shotshell holder together to help with quad-loading during a 3-gun competition. 
When there are shells in the holder, the “cups” are held out from the shotshell holder, making them easy to grab. The cups fall back down against the back plate after the shells are removed, allowing better access to the lower row of shells. No hardware is required for the default version.  Supports are required to print the backplate and the cups, and must be cleaned up for the cups to rotate properly.

I printed two holders in FLASHFORGE ASA Sparkle Black Green, which has held up well.

Note that other filaments (and certain types of 12g shells) may not give the right amount of retention on the shells. I suggest scaling the cups to a size that fits the shells you use most, then scale the whole assembly by the same amount.

I did make another version of the backplate which can take 4mm heat-set inserts to attach a QLS fork.