This looks great, but it looks like it'll be a bit of a hassle to generate the layer yourself. If KiCAD gets a "generate heating layer pattern" feature, this would become trivial.

Then again, I don't know how much I want to bother with masks and placing things myself, given how cheap JLCPCB's assembly service is.

I wonder if it'd get hot enough to print this on a separate board and just lay the working board on top.