This is one of those things that may sound totally ridiculous until you try it, and then once you do you'll go around thinking, "This is the best wallet EVER!"
The humble binder clip (also known as banker's clip, bulldog clip, or foldback clip) was invented in 1910 by Louis E. Baltzley, and in my opinion should be recognized as a marvel of industrial design. Seriously, it deserves its own pedestal at MOMA. It is so small, strong and flexible and it securely holds as
little or as much as you'll ever need. It is very easy to get cards
and money in and out without spilling the other contents, and nothing ever falls out.
I switched from wallet to money clip just after college. I was doing a lot of international traveling, and wanted a slimmer and more discreet way to carry my cards and money in my front pocket. A couple of years later, my fancy silver money clip broke, and I grabbed a binder clip off of my desk to use as a temporary wallet. That was about twenty years ago, and I am still using the very same binder clip.