The tip used in the soldering iron should be smaller than the slot desired. the tip is so hot, it will burn off the foam without actually touching it. This has a few undesired effects. The width of the slot varies a bit depending on how fast you move through the material. If you stop or hesitate to reset your feet, you’ll be left with a large defect at that point. I cut the slot in the pictures all in one go, as starting and stopping creates relatively large defects. Plan ahead and start and stop in straights, and make sure you wont have to reset your feet in a curve.
I started with a 40W fixed power iron, but that was too hot. I used my adjustable iron in the final iteration. I don’t have a lathe, so I just built up the cutting tips by nesting K&S brass tubing. I tested 2mm, 2.5mm, and 3.0mm diameter cutters and various wattages. I found that 2.5mm and 30 Watts was the best. I wanted the slot a little oversized, as I was planning on a few extra steps to be described shortly.