What type of foam?

I'm rebuilding a heater box but don't need every seal and foam sheet in the "heater box foam kit" that DMT has for $80. So, I've decided to make the couple of charcoal foam door sheets I need.

Do any of you know what type of foam is used for the internal door in a heater box? I'm assuming some sort of light weight 1/8" thick open cell stuff?

Thanks!