Ya, that may work. I’m not sure about the form 3L platforms, but the regular 3’s are just extruded aluminum with plastic caps on the side. Put it in a vacuum chamber to pop them off. I live at altitude, so the atmospheric pressure does that for me whenever I order them. You could try just epoxying a few large magnets in the cavity against the working face.
Regarding machining the surface, I don’t see why not. You could probably deck it down 1-2mm and still have plenty of meat to work with. The build platforms don’t have to be deadnuts flat, the $250 stainless steel ones I just bought certainly aren’t. I can put a flat reference across the surface and easily slide a 0.005" shim in there. It’s a concave deflection btw. For a sheet, I would go with a 301 stainless spring steel. Spring steel so it can handle the flex cycles and not distort. Stainless to resist any interaction with your resins. This wouldn’t be a bad place to start: McMaster part: 2416K129. I think this material is magnetic - definitely try it before committing to modifying your build plate. McMaster says the material is cold worked, which typically means the magnetic properties come back - but not all stainless steels are.
For reference, I just measured my Prusa flex plate and it’s 0.05" thick. It feels pretty chonky, so you could probably err a little bit thinner - I think the Prusa plate is coated with something anyway. I’d sandblast the front or hit it with a sander to rough it up to promote adhesion.
Also, the build platform thickness on the Form 3 is 35mm, I’d imagine it’s the same on the Form 3L. Just machine down whatever thickness of the spring plate is and, in theory, you should be golden.
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