I came across a few designs for money clips, but many had the same problem: they didn't grip so well. Also, many were thicker than I wanted and would be too bulky in a pocket.
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).