Rad tanks

I've got a copper rad that came out of my old W900. Core looks mint and isn't leaking but both tanks are pretty much toast. Looking online seems like I'm looking at a minimum of $600 to replace the tanks. I figure I can build new ones cheaper. The only real snag I'm running into is what material to build them out of and what to coat the inside of the tank with. Thinking maybe I'll use 1/8" mild steel plate. Not sure if that's a good plan or not.

Rad tanks