Small Cans and cup holders? Any inserts?

Try google searching something like red bull can adapter cup holder...
Alternatively, you could also take some measurements (or the can) and go to a hardware store and look for some PVC (or whatever material) pipe with your desired inner/outer diameters..
OR, you could go low cost/tech and make a good enough adapter with some cardboard and duct tape...

Something along the lines of the WeatherTech Cupfone would be great if you could get those cups only. One other thing that comes to mind along those lines are measuring cups that come stackable (fit inside one another) only thing w/those is most will have a handle.