i made this adaptor to mount a flat base ebike battery on a round frame tube.
designed to add stability (hopefully). made to fit the 51mm diameter diagonal tube of my bike, should be ok for other bikes with some rubber to set it tight. also there is a groove inside the part to secure it around the tube with a hose clamp collar if you need. This thing was made with Tinkercad.

still have to test it, but it should do the job. testers and comments are welcome.