While the StereoPi2 (which takes a RaspberryPi 4 Compute Module) is not yet available, there are other CM4 boards with 2 MIPI CSI connectors which we could use for Stereo Ninja. One of them is the official CM4 Eval Board. I now have another one in my hands (thanks Tim for borrowing!), which looks perfect: the Waveshare CM4 IO Base Board. It has the needed 2 connectors, is very compact and only costs 20$.
While the Raspberry3 (and StereoPi) doesn't support FullHD in Side-By-Side Mode, the CM4 should be able to do that, as it even supports 4K output. So we definitely have to look into this!