[JMay]

Like this, a main boss with a spoke rim and 1-2 spoke rims that slide over the boss and can be rotated to offset the spoke pattern and as you state are flat to the printer bed. There is a small offset so the next rim stands off from the preceding one. Spokes are 0.3mm thick and 1mm high so that the insert has some strength, but you could reduce the height down to @0.6mm or lower depending on the spoke length. 

   

Cheers
          John