I guess sq wave is ok lang kapag gagamitin sa ilaw, heater, at switching supplies gaya sa PSU ng computer natin. Sa PSU, e DDC rin and supply nya anyway.
Pero Brains, sa US may nabibili ng 12V soldering iron gamit sa RC hobby. You an make a 12V soldering gun but you need a high current regulator. The tip is similar dun sa Weller tip. Pwede ka magpagawa ng single cell LA battery and use 2V with a series nichrome wire to tune the heat.
Kung matiaga ka lang,bumili ka ng element ng 100watts(ng soldering iron). tap mo with 12V sa tamang turns, iballik sa housing. You need a big wattage cuz after reducing the turns, hihina ang watts yan.
I would go for the gun type. make a hi current voltge regulator using the Lm317 which can go to 1.25V.kaya lang mataas rating ng bypass transistors..one 3055 or 2955 can handle 4 amps...kung 20 wats lang divided by 12V, that is 2 amps more or less. Use 4 transistors and heatsink.