I have recently become fascinated with fountain pens, and have already acquired 8. I also have a rather fancy wooden desk pen holder for 9 pens. I can see the time coming when I will need more storage. So here is my idea:
Small wooden bases, for modular addition as more pens arrive. Sections of PVC pipe (1" external, that has 13/16" internal diameter seems to be the right size), cut on the base end with maybe a 5 degree angle so that if this gadget is hanging on the wall the tubes will have a slight rise and pens won’t be tempted to fall out. Not sure if I should leave the outside end of the PVC tubes square or use the same angle there. Bore out the outside end of the tubes a little to remove the sharp inside edge. Make a jig so I can use a router to rout out exactly the right size holes in the base and give more glue area; set the pipe pieces into the base with glue (which I hope will be enough strength).
Arrange the tubes on the base at an offset rather than inline so they can be closer together, maybe 3-2-3 for the first base. There would be two shapes of bases, one for 3-2-3 tubes, one for 2-3-2 tube arrangements, and the two shapes would nest together.
Things I haven’t figure out yet: cushioning of some kind inside the tubes, so that the pens don’t get scratched? I can probably figure out some way to buff the insides smooth, but they’re still hard, and fountain pens can be susceptible to scratches. Not sure if this is a concern. Also, some structure into which I can slot each new small base as it is finished/needed. Or some other relatively seamless way (where the end result looks good) to add new small bases over time, where I don’t have to take the existing ones down, take all the pens out, and attach them together some way before rehanging the whole shebang.