You're right about the thickness of the bottom plate. It's about .170" thick as is the top plate and that large nearly 5/8" spacer that connects the two plates has to heat up as well. The worse mould I ever cast with was a 510 gr mini ball mould with the bottom cherry. It had to be as hot as the mould or you'd get wrinkles and voids. Using pure lead as was needed it took a long time to get just one right. I abandoned that idea and just bought them when needed.
I can't fathom -30 degrees. I'd freeze to death quickly being on blood thinners. There's not enough coats to keep me warm in that climate.