I got:
- double-glass (two singles used together)
- have my box very well insulated
- not much extra air to heat by design (the box is flat)
- the tray is from a very dark and thick steel sheet I picked up by the roadside.
- multi-purpose in function.
No complaints (outside of being burning hot; one glass pane already cracked from it).
Works very well for everything.
Be aware that very dark combs will not melt completely because they are, essentially, a mesh made up from bee cocoons.
However, they should fall apart into separate cells.
Beyond that not much else you can do.