Magnetic Keeper

Print 2 of each object for a complete magnetic keeper. This is similar to the ones you see at the fly shop for $10 to $30, but those are usually made out of fancy machined, anodized aluminum.

Requires 2 12mm x 6mm (1/2" x 1/4") neodymium ("rare earth") magnets (usually about $1 each), and 2 split rings (pennies if you even have to buy them... I have dozens of them from various giveaway keyrings).

There are several online stores that offer the magnets, usually 10 at a time (so you might have to make 5 of these and give some to your fishing buddies).

Notes:
* The threads are designed, approximately, as 5/45 buttress threads since they're a little stronger than typical 60 degree threads in plastic. As a result, the overhang on the printer is a little more harsh on the cap. You may need to clean up some threads with a hobby knife if they sag a bit.
* There is a tiny bit of extra room inside to accommodate magnets that might be slightly larger. If you're bothered by the rattl