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In order to get it really strong, I recommend to print vertically, with supports (see screenshot from Simplify3D)
0.2mm layer height / 0.4mm nozzle
30% full honeycomb
4 outer layers, 4 bottom/top layers
I was also considering printing it vertically for strength reason. I'm glad I found your make because everybody else seems to have print them flat.
Since you printed this about 10 months ago, can I ask if it's still holding? (I'm afraid it could delaminate around the screws holes with time)