I'm looking for recommendations for a body/hidden wallet. I want one that pins to my side. I'd wear it over my undershirt and under my overshirt, and I'd put my phone in it and a bit of local currency, maybe my passport. Ideally as thin as possible and pins tightly to body (doesn't just hang), no frills if they compromise this first principle, though waterproofing and RFID blocking would be nice.
Consider as an example this. It has most of the features I want, but it'll just hang from the shoulder. It needs another fixing method to pin the bottom of the wallet too, but I can't think of what that might be. It's probably also bulkier than it needs to be based on the description. Throw your suggestions at me.