I've been fighting on and off with issues with my build plate seemingly not being perfectly flat. I've ordered a machined straight-edge and some new feeler gauges to diagnose the problem (and test any potential replacement) better, but I'd like to get an idea of what options are available for obtaining a flatter build surface. I'm not really interested in approaches like BLTouch since I want accurate flat bottoms on my prints, not just consistent adhesion.
My printer is an Ender 3 and I believe my problem is a mix of wear to the Buildtak clone surface and warping in its backing plate. Mine is the newer model with removable buildplate and clips, and the backing is reportedly a "glass fiber" (FR-4) material that's hard to find genuine replacements for.
Glass seems like an obvious option - I might even put a buildtak clone on one side of it and use that most of the time - but I'm somewhat concerned about weight and whether having glass moved by the Y axis is going to limit acceleration. I've had (seemingly unrelated) problems on and off with layer shifts, which seem to be mostly Marlin's fault not mechanical, but I'm scared to introduce another factor that might encourage them.
Are there other non-glass options I should consider that would provide a rigid, flat backing? I'm very happy with the buildtak clones and their adhesion properties, so my leaning it to look for something that makes a good backing for them rather than a material that's intended to be built directly upon.
Measurement results: Using a machined straightedge and feeler gauges, the surface seems to be at least 0.10 mm but less than 0.15 mm lower within a couple inches of the bed center. This is plausibly just wear on the buildtak-clone surface, so I think I'll try to see if I can get by with just replacing that for now. But I'm still interested in the general topic of the question.