I updated this so that there is plastic between the screw and embedded nuts- an earlier design had the nuts on the same side as the screws and the nuts would climb up the screws and become un-embedded.
This design needs improvement, because tightening the belt is difficult- it has to be done from underneath the reprap. I found that the embedded nuts would sometimes be drawn up out of their bed. Using m3x16 tapping screws works, but probably not for too many belt changes/re-tightens. It needs the embedded nuts to have plastic between them and the screw like the old horizontal x-axis reprap mendel design.