If you're anything like me, you carry some change in your wallet. Say you need to go to the vending machine but all you have are $20s, spare change can help a ton. There's only one problem. Unless your wallet has a zipped up compartment, your change will be spilling out before you can get into McDonald's.
Having suffered this problem myself, I wanted to come up with a solution that is permanent and looks really nice. That's why I made the CoinKeeper. It's just a little 3D printed part that you glue to a spare credit card, and it works great. It fits in any wallet with a slot that can hold around 6 cards (~4.7mm after adding card).
Step 1: Preparing Card
First, to prepare the card, simply take it and apply duct tape to both sides. My tape was slightly too thin, so I had to use multiple pieces. If you do have to do this, try your best to line up the sides so they match up evenly. Snip off the excess with a pair of scissors. Once both sides are coated, you're ready for the next step.
Step 2: 3D Printing
Here comes the exciting part - 3D Printing! Here, you'll have to download the STL file I've included. I printed mine in FoxSmart Aluminum PLA @ 190°C, 20% Infill, and 0.3mm Layer Height. This took about 10 minutes on my mechanically stock Anet A8 with a print speed of 60mm/s and travel speed of 120mm/s. As for the model, I designed it in 123D Design using dimensions from the ID-1 standard. It is roughly 3.9mm thick.
Step 3: Finishing Up
With the part 3D Printed and the card ready, It's time to put it all together. I recommend using binder clips for this step, as it makes it much easier to keep the parts aligned while putting them together. Using a high-temperature hot glue gun, apply glue to the 3D printed part and stick it down to the card, applying pressure for about 30 seconds. Do this for either two or four sides. When it is working the way you want, try it out!
Put it in your wallet. There should be a decent amount of resistance when you try to put it in your wallet. Once it's put in your wallet, you're good! You can now try putting some coins in and closing the CoinKeeper. Shake your wallet around - nothing should come out. You now have a working coin drawer!