Small and long hexagonal 5.5 angled hex wrench/spanner. Designed explicitly to adjust the U bolts under the MK2S/MK3 Prusa bed. Printed with PLA with 5 perimeters, the key more than strong enough to do the job (the bolts shouldn't be over-tightened).
Inspired by https://www.thingiverse.com/thing:2766241
The angled front allows adjusting the rear bolts much more easily. I had trouble with some bolts with SubSite's shape, and of course any metal key will be painful to use under the magnets. Just cut the long handle to size if needed.
I recommend 5 perimeters at 0.2 layer height for optimal results. Infill doesn't matter.