What are your thoughts regarding safety on this equipment cable entry?

You could make a plate 35mm high with two holes in it (as AJJ said) which could be threaded to accept the stuffing gland without the nut. You could use the nuts on the outside, flat to flat, to give a bigger footprint than the gland flange might have, to hold on to the box better.

Or holes with grommets, but I would then be looking at cable restraints drilled and bolted into the flat plate below. 

Any such modifications might need the manufacturer's approval, but it sounds like they haven't really thought about it so might welcome solutions.

This could well be a good way to go.  Might just fit 2x H07 into a 6mm aluminium plate - thick enough for decent level of threads. 1x S/T screw in each corner would hold the plate in place...

This could well be a good way to go.  Might just fit 2x H07 into a 6mm aluminium plate - thick enough for decent level of threads. 1x S/T screw in each corner would hold the plate in place...