To get a properly flat print surface, one option is to replace the aluminium print plate with one made to higher precision, from cast aluminium. Cast aluminium has a big advantage that it warps very little with temperature changes, compared to extruded or rolled aluminium, due to lower internal stresses.
WARNING: newer revisions of the Z-603S have been reported to have the heater PCB stuck to the aluminum print plate. Make sure you can actually remove your print plate before you buy a new replacement print plate!
Download the designs for the new print plate here.
More info on my blog… https://samuelpinches.com.au/blab/aurora-z-603s-new-buildplate/