I designed this 7 shell holder, as my KSG takes 7 shells per tube. you can fit 10 in a Plano Field Box, as well as the Harbor Freight (Bunker Hill Security) equivalent. As some shells are sold in boxes of 25, I also included an x4 holder and x5 holder. 

If storing 1 box of 25, 3 prints of the x7 are required, as well as 1 print of the x4. 

If storing 2 boxes of 25, 7 prints of the x7 are required, with 1 loose shell

if storing 3 boxes of 25, 10 prints of the x7 are required. as well as 1 print of the x5

Initially I used PLA, though I have a feeling they will warp in a hot car, so I may re-print in PETG