Low idle/running current is one of the major reasons I fell in love with the 'oztules' OzInverter.
My first Inverter was a APC UPS 5000, and its idle current, was at 150 watt to 200 watt, so overnight my batteries lost nearly 2kw, arrgghh.
My Old, now returned to SMA, Sunny Island 6, had I think separate windings for getting the idle current down, as at night there seemed to be a battle happening at night time as the lights flickering on and off.
Here is a pic from the book regards the ecore.
That's 50mm/2, Tri-rated/multi strand/car battery/welding wire, it comes in different insulation thickness and can be easily shaped to fit the ecore.
I can get 4 turns, but looking at the 60,000 words on the OzInverter it seems that several reports mention NO particular benefit of 4 turns, so 3 are best. The needle file is just to very lightly/gently remove the very sharp edges on the ecore, as they nip the insulation if you don't, see below pic.