I think that the RS connector had similar size as the element14 and Altronics. I may even have them all here.
Question, how many connectors do we actually need to go into the box? USB + just one voltage (and GND)? Is everything powered from this voltage internally?
Edit: What I meant is that if PWM for the Peltier is generated inside then I don't see any issue passing USB + voltage through the same plug.
Something like
this or
this may be suitable.