LOL,
I only took one thermal kinetics class (easy stuff btw...), but I'll take a stab at it
Firebrick is obviously an insulator. Essentially, any radiant energy from the fire contacting it goes nowhere. You want to reflect as much radiant heat back into the room as possible.
A fireback simply acts as a reflector, reflecting the radiant energy back out. Any reflective, conducting surface will do. Obviously, it must take the heat.
A simple, thick plate of iron or stainless should do (say ~ 1/4" thick). Just make sure you rig it with some kind of stand. I suppose you could take thinner sheet and bend it to be self-standing. Any cheap stainless alloy will do. Bend it like this \_/
Don't bend it square. You want the heat to radiate back into the room.