Bed design for magnetic easy release when printing PLA
Follow up:

I used the 6mil magnet, cut a sheet of thin steel, (about .5mm) to fit, added a layer of Buildtak and now it's much easier to remove prints, build surface is very flat. I assume you could deform the metal beyond the magnet's ability to flatten it but it works quite well. The metal was cut with sheet metal snips, risky as they can bend the sheet. Probably better done with a saw though.