Again big thanks to Joe Balint for suggesting to simply remove the corners for easier printing! This way I was able to crank the resolution up to 100x100 pixels and have it still sliceable in Simplify3D!
Make sure to have no important detail on the corners, because it will be lost. How much detail is removed can be adjusted with the setting "corner cutoff", the default settings worked fine for me with printing at 100%.
Printed in 18h 16min at 60mm/sec.
The print is not perfect, probably a higher resolution or slower speed would give cleaner results.