I've never done what you want to do, so take this with a grain of salt, but what comes to my mind is to make a "clam-shell" sort of forge, where there's a bottom half and a top half, and in between them is enough space for you to get any particular part of the armor in there to heat it. Or maybe even an open-top forge, where when you put the armor over the top of the open-topped forge and the armor itself holds in and focuses the heat.