Magnetic Keeper

thingiverse

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 rattling, and if you don't want to just epoxy the magnet in, you can fill the extra space with baking flour or talc. https://youtu.be/711mGjhFdeg