In my perpetual quest to carry ever more stuff without panniers, I'd like to fit a second bottle cage under the downtube, for small bottles (e.g. 200 ml of stove fuel). There's space for something small, as my tourer is massive with relaxed geometry, but even an adjustable bottle cage like the Topeak modula EX won't quite fit (and won't go thin enough for such a small bottle). The shapes of the Salsa Anything Cage and Topeak Versacage are ideal, but they're bigger than standard bottle cages, not smaller. The mounting hardware to use the versacage without threaded bosses also adds a lot of standoff, unlike cheap bottle cages that are designed to fit with cable ties or straps.
I took this photo for something else, but it illustrates the available space under the downtube and above the toolcase.
So, assuming there's nothing on the market, how can I make something, perhaps starting from a bottle cage or even a pump mount? It needs to be pretty robust as it's hanging under the frame, and wants to carry a bottle around 50 mm in diameter (about 2/3 of a standard water bottle).