A simple case for two Airlinks (https://www.neorblab.com). I don't know how many people actually have these, but if they go on sale again, I highly suggest getting one. The grove around the design is meant to be for a rubber band to hold the cap on. When you clip Airlinks to things, the back clip tends to bend out a bit. I used that to make a great friction fit in the case that doesn't damage the Airlinks at all. The bottoms were flattened to allow supportless printing.
Printed with default Prusa i3 mk3 settings (0.15mm OPTIMAL MK3) settings, except for five layers of raft material. When printed with no raft, it tipped over mid print and got filament all over the extruder.
If you have any damaged Nerf darts laying around, I would suggest gluing two 10mm thick sections of cut foam to the cap to keep them from moving around too much if you drop it. When I made this, I made the hole for the Airlinks come too close to the outside of the shell, which results in a thin slot in the very bottom of the base. I didn't correct this because you can cut a rubber band in half and stick the ends into the two slots (with superglue) to hold it on better. You may have to expand this slot a bit with an exacto knife.
Rubber band slot