At the moment, i have a solid core door supported by some pieces of engineered floor joists leftover from a job site I was working on. The joists are very strong and the legs are made from 2x4 and 2x6 pieces screwed together in an a L shape. Its very sold, just not very flat. I was thinking of either getting a frame welded out of 2" angle iron or 1.5" square tubing, or possibly shimming my current setup to make the top perfectly flat.

I guess the metal frame is not as widely used as I thought it would be