I have a 510 pcc and was wanting the same thing. I have a 3mL rubberbanded to the case, I wanted something to fit inside it. A friend of mine suggested, after much debate as to whether it would be useful to carry
juice (he fills his carts and carries no
juice), a small plastic/glass perfume sample bottle. It would be the perfect size I believe, and I cannot think of anything else that would fit without modification.
Though the question is... is it worth it to carry around 1mL of juice in your PCC vs. carrying around a full cart? You wouldnt hold much less than 1mL in a 510 cart with PTB and you wouldnt have to deal with filling a cart with a bottle that has no dropping spout.
Hope some of this information is useful. FYI: I still rubber band a bottle to my PCC instead of filling my carts (I have the extra carts blank with PTB) and am currently in search of a pouch to carry my PCC + full eCig + some juice + etc. Nothing fruitful yet (found one at wal-mart I liked but they wanted sixteen frikkin dollars for a small pouch. AND it was purple... f that)