OK then, let's get started.
This Instructable is a part of the 3D Project Challenge: Money Clip. Create your own 3d file for a money clip or modify the template to enter.
Step 1: Design issues
Tinkercad.com is great ant their tutorial for a money clip is great for learning, but the results weren't so hot once I printed them. Too big and bills easily fell out when I printed it on a MakerBot Replicator 2.
So that leads to two things to fix. The size (no problem) and the grip (a bigger problem).
Step 2: The grip
To solve this I created a dip in the body of the money clip. Then the arm of the clip could extend past the original surface of the body. With this done, the money would have a lot more friction since it would have to go around the arm. This also gives us some tension since the default position for the arm is past the body.
The result? Money stays in nice and tight. Even with one bill folded over it held strong, keeping the bill in place while it was being moved or shaken around.