I bought a 200W Kat's magnetic oil pan heater for this winter.
It's supposed to reach temperatures of between 300º-400º.
I've read in a couple of the posts here that it's suggested to use RTV to help hold the heater on the pan.
I've got some Permatex Ultra Copper high temp RTV.
There is a small gap around the edges of the heater when I put it on the pan, the magnet sticks out a little further than the base.
Would the Ultra Copper RTV be ok with those temps and do you just put a small bead on the base around the edges to help hold it on and seal it up?

 

I think the Ultra copper won't have any trouble with your temps. It's also supposed to have great adhesion as well so I think you're on the right track.