The kobuki base alone won't drift, so it is sensible the tilted inertia results in the drift. Not surprisingly I can hack my URDF using a weightless superstructure, but I wanted to fix this in a far more realistic way. I tried all I can think about, and all failed: https://www.uk-exhibitionstands.co.uk/product/pop-up-stand-3-x-3-quick-plus/