How can I DIY a smaller Anything Cage/Versacage, or a mini bottle cage
There is an option on the market as well, in the Specialized Burra Burra.

While the cage itself doesn't really 'hold' anything, I have one and find it super useful for strapping in any range of item sizes (water bottles, MSR fuel bottles, growlers, etc.). I didn't love the straps it came with, but a cheap ski strap (e.g., this one ) works perfectly. Anything cylindrical stays seated very well against the aluminum tubing of the cage.