Thank you for your replies.

I ended up with this




... but my question still remains. As the plate gets hot from the stepper, assuming a 50°C temperature change between assembling and max. operating motor temp., the difference between aluminium and steel (bearing) expansion will be 0.02mm. As I don't want to end up with play I need to use heat mount but I'm not sure if -0.05mm for the hole is fine or should I try with more? My only concern is the elasticity of the aluminium..