For my Cherry Pi i have been using a granite chopping board from ebay here
The unpolished surface is the flattest. Using feeler gauges and a steel rule I guessed the flatness to be around 0.05mm. Cheap as chips too.
The unpolished surface is the flattest. Using feeler gauges and a steel rule I guessed the flatness to be around 0.05mm. Cheap as chips too.