I got the idea for this from the container by "Zombie Batman" (linked as a remix, though I did model it from scratch to fit my purpose)

It fits the Pinecil case made by Termiman (also linked as a remix to make it easier to find), or at least the specific variation that I printed (see my pictures, it won't fit all of them)

Please note that the lid of the flux container does extend above the surface of the Pinecil case, but it works because on the opposite side I have the solder cleaning wire. It would also work with a sponge there. The wire/sponge compresses against the lid of the Flux container and holds it in. Make sure you're printing one like this and that you have a sponge or wire cleaning pad to put there (otherwise it will rattle around)

The lid snaps on quite tightly - maybe a bit too tightly. The easiest way to snap it off is to spin it 90 degrees so you can get under the “wings” easier and pull both of them evenly. It may loosen up over time.