Done! I just received an used CPL from ebay. The Quantary CPL has flipped glass, and the Hoya one is unaltered. The step-up ring and larger filters prevent vignetting.
100% Crop of a shot of the sun. Notice the bluish tone --- the whole ND filter looks blue at its dimmest setting. Is that because of UV rays??
A test shot of flowers. With my A50/1.7 at f/2.0, I could never use the in-camera flash to reduce contrast, because slow-sync limited the shutter speed to 1/180s.
Same settings, with my homemade variable ND filter. White balance is fixed at "flash" in both shots.
I own the Hoya CPL already. For a 3 dollars extra, I got a variable ND filter! The results are a bit bluish, but that can be fixed easily in Lightroom when I shoot in raw format.