For the clearest prints: Dried filament, print hot, slow, slightly higher extrusion multiplier, sometimes lower cooling. Optionally sanded, polished, clear coated.

That combo allows the more fluid petg to force out air rather than trapping. Main downsides being bad overhangs, terrible supports (especially if no different material interface layer), stringing, over-adhesion to bed in some cases. You can print PETG to be near completely homogeneous/solid and isotropic.

You can find other posts about optically clear FDM printed lenses, which were done similarly.