When I was young I used to disassemble things and, occasionally, reassemble them. I made weird gadgets out of legos and I've continued breaking things and sometimes fixing them since then.
This page is a catalog of some of my projects,
Years ago I used a money clip with a card holder but even that was too bulky so after several iterations I landed on this:
It is a card holder which is designed to hold 4-5 cards and one or two bills of currency. The bills are folded underneath the cards and act as a spring,
thus keeping your cards from coming out but allowing relatively easy access. I only rarely need the cash but having it with me is worth the minute extra thickness.
If you want to make one yourself, the link is here.