PCB standoffs. For best results use .05mm layer height for threads. this will give you 10 layers per thread and should result in good performance. Please let me know if you find them useful.
12 mm standoffs with M3x0.5 threads.
3-10-19 - Updated with chamfers in threaded holes and looser tolerance on threads. Works much better now.
The threads are a little tight when they first come off the build plate, but they work loose easily and then work freely like they should. Your mileage may vary of course. I printed mine with a 0.4mm nozzle but I suspect you could get much better results with a smaller one.