Door speaker maximum depth?

I just upgraded my door speakers on the weekend and seem to have encountered the same issue I've come to understand many others have as well, with the corroded wiring. I'm well past that now, but the thought occurred to me that with moisture getting down by the speaker and corroding wires, I might try and find a 6 1/2 speaker baffle to protect the back of that speaker.

Without taking my door apart again, I'm just wondering if anyone has happened to have taken measurements from the metal surface to the glass to figure out what the max depth might be?