I built a custom testing rig out of some plywood sticks and these kind of elastic cords:
There is just enough clearance for 7 inch props and the quad is free to rotate around one axis (Y or roll in the video). I can easily change the free axis as the cords connect to the arms with metal hooks.
The issue I’m debugging now is related to a tendency of the motors to generate different thrust for the same throttle input. I think it’s because the ESCs have different settings on them as I couldn’t find any issue with my code so far. If I send 50% throttle to all motors the quad spins a lot.