we have to differentiate here:
do your Konstrukteure just draw the diagramms, based on designs and rules and regulations given to them from, say, a designer, or engineer, or design engineer, or do they actually design the things themselves, and drawing the plans just belongs to their jobs?
if the first, then they are 'draughtsmen', if the second, then, again, it depends on which field of work they are designing in.
they could be engineers, of any of a whole plethora of different types, from construction engineers, to machine engineers; or designers assisted by engineers, as in motor car design; or architects, or static engineers, or whatever else....
if your Konstrukteure are qualified engineers, then that's what to call them.
if they aren't, but the context of your translation is casual, then you could call them engineers, or perhaps technicians.