Wow, that's an excellent idea. One could actually put the fountain pen in there. It would protect the finish, protect the rest of the luggage from leaking ink, not take much room while being light weight, and it would look cool.

As you say, these would also be very nice for shampoo etc. I can imagine tons of uses. Perhaps even as a flask for some whisky. In terms of fountain pens they could also be used to actually store the ink while on a trip. The fact that they are of small diameter means little to no ink will be lost when filling up the pen.

Very helpful post. Thanks! ^

Till