I have an old door that is close enough.

I've thought that if I really wanted to get a perfectly flat and level surface, I would build a small frame around my old door such that a self leveling floor compound can be poured to a depth of a inch or so. That would create a perfectly flat, level, surface for little cost.

I'm not talking about the stuff that has to be trowled flat. I'm referring to the liquid slurry stuff that finds it own level.