Magnetic Keeper

thingiverse

Print two of each object for a complete magnetic keeper. This setup is similar to those found at fly shops for $10 to $30, but they're usually crafted from fancy machined and anodized aluminum. Requires two 12mm x 6mm (1/2" x 1/4") neodymium magnets (also known as "rare earth" magnets), which typically cost about $1 each. Additionally, you'll need two split rings, such as pennies if you have them on hand - I've accumulated dozens from various giveaway keychains. Several online stores sell the magnets in packs of 10 at a time, so you might need to produce five of these keepers and share some with your fishing buddies. The threads are designed to be approximately 5/45 buttress threads, making them slightly stronger than typical 60-degree threads found in plastic. As a result, the overhang on the printer can be harsh on the cap, potentially requiring thread cleanup with a hobby knife if they sag. There's a tiny bit of extra space inside for accommodating larger magnets. If you're concerned about rattling and don't want to simply epoxy the magnet in place, consider filling the excess room with baking flour or talc.