From your post, I didn't understand (or missed) which particular model Coinplate did you choose? Alpha, Grid or Punch?

How about putting some material (rubber, silicone or something completely different) between the two plates for additional protection (at least around the perimeter) to prevent corrosion?

p.s.
~$300 is way too expensive for a simple piece of metal, even if it's stainless steel, and with bonuses like a couple of bolts, a marker, and other stuff that's sold at any hardware store.