A 60 amp mosfet will give you square wave.
I used to just use my Weller 240 w soldering gun with the tip sharpened to a needle point for fine line writing. and various other tips ground to what ever shape I needed like a ball end or a flat spade end
Are you going to re wire the secondary of a microwave transformer for your power supply? If so add a variac ( variable resister) to the primary side of the transformer this will help control the output heat and voltage
Use High temp furnace wire probably 12 or 14 awg for your leads to your pen Use 14 or 16 AWG nicrome for the burn tip