I think I had good success with the “initial frame” because the whole thing was supported by the motors in a good/stiff way (adjust the base so that both motors are holding the board roughly equally):
And when you mount the motor mounts, I used 4 screws… but laid them out so that the motors couldn’t move ('course that does mean you need new holes if you /want/ to move the motor) – I did this after one of my motors cocked sideways causing the chain to fall off and drop the router during calibration:
Permission granted to reuse/modify/etc. And please ignore the mess in the background…