There are a variety of portable soldering irons. Some use a fuel gas; others use electricity. I have attached images of two electric versions. The larger (Sears) unit is mains powered and is just like the one I use for cutting plastics.
I use a piece of heavy wire for my cutter that has been flattened in the center to cut plastics. The problem, of course, with a portable unit is that battery capacity is not adequate for extended cuts. If I needed longer cuts, I would use a cutter based on fuel gas.
As for current/temperature regulation, there is some regulation from the source with AC powered cutters; however, most of the regulation comes from the user and how fast you move the wire. I would just find a wire of suitable resistance and make fine adjustments with the speed of cut and by pulsing the on/off button.
That is exactly how I control my mains-powered portable iron. John