Having used screwdrivers and other improvised tools to clean out various 3D printed recesses, it finally penetrated my thick consciousness that a boring bar is exactly the right hammer for the job:
In normal use, a boring bar’s head cuts mainly on its end surface, with the side cleaning up the hole’s periphery. Those edges remove droopy threads and Reversal zittage around a hole’s interior; an end mill works better to make the recess uniformly deeper.
I have a few sets of these things, with larger & smaller cutting ends and longer & shorter shanks, that I occasionally use for lathe boring and rarely for mill boring (in the manual mill, not the Sherline!). The smallest head in the collection is maybe 4 mm across, so there’s a definite lower limit on the size of the hole they’ll clear.
Hand-held while cutting plastic? They’ll last forever!
I should probably print up some handles…