There are flat KR808D-1 cases with diagonal or horizontal rows of depressions to hold batteries or cartos but the cases are pretty heavy metal with decorative faux-leather sides. Pretty heavy. You can lay cartos down in them with the condoms on. The PCC is the same but a rectangular battery takes up 1/4 of the space so it holds one battery for recharging and 2 cartos.
I have a 510 too and I use a Vapormaxx PCC (cig-pack-look) for it and it has six slots that will hold 510 and KR8 cartos with the condom off. I worry about drips.
You might be able to find a cigarette case that would hold cartos but 510 and KR8 carts are a bit thicker than a standard cig.
I have been using the pocket of my sweatshirt cardigan to hold
juice, paperclips, carto condoms, spare battery with a condom on the threaded end etc. I better think of something else to use before it gets hot out!