Best configuration for 400W resistive heating device
Personally, I would (if possible) have a groove milled into the disk and use a custom tubular heater that runs from the mains. I'd specify the heater at about 1.5x the power I think is required. 

Tubular heaters are like the elements in a domestic oven- swaged with powdered ceramic inside- and are very long-lived and robust. 

Easily controlled using an SSR- 500W would only be a few amperes.