Resized to be a little bit smaller, completely enclosed, and to break in the middle without tools. You will have to edit the G code that your slicer puts out to pause printing at the right moment (one layer before the inner cylinder is closed off) to place money inside.

There is no way to put money in after printing.

The only way to get the money out is to break the keychain in half.

10-2-2018
Added a smaller, weaker, version of the keychain. Basically, my mother asked for one, but there was no way she could break open the original. Stomping on it is out; osteoporosis. I probably printed 30 of these trying to get the break strength just right. Strong enough that it won't accidentally break in most circumstances, but still possible for someone without a lot of hand strength. As a result, it is a little bit smaller than the original keychain I made.

I printed slightly hotter than normal to ensure good adhesion between layers.

With 2 shells and 25% infill, it takes a considerable amount of force to break this open. Children, the elderly, or anybody with weak hands may not be able to break it bare handed. I suggest printing a test piece and having the intended user test for break-ability, and modifying your settings to make the piece weaker or stronger as needed.

In any case, you could lean it against something and break it with a foot if you don't have the hand strength.