Like seconds after my reply I did find something that might become my solution:
The XTR110 from TI together with a precision 250 Ohm resistor, PNP transistor, and a secondary power supply.
See the datasheet of the XTR110 for an application note.
This will make a 15-30V 3 wire 4-20mA sensor.
Again I'm only doing a prototype and don't care at first about power supply.