M3/M4 Nut Wrench
thingiverse
This nut wrench is hollow, which allows to tight nuts even if the screw is too long. It is 3 cm's tall, I use it to tighten the nut in the X axis tensioner. The M3 might be a little bit to tight, but scaling it to 102% should have enough clearance to tight the M3 nut.
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