Where are you located? I bet buying a 3-3/4" hole saw would cost less than shipping one two ways.
I used large slip joint pliers to tighten the nut rather than pay for the single-purpose wrench. For really tight spaces, you can drill two shallow holes in the nut and fashion a two pin lever with roll pins and a length of flat bar steel.
Central VA. I do have a monster set of channel locks. I may have actually bought them for this, 2 boats ago. Yeah, dang the saw is only 17 bucks on Amazon, and I have mandrels.
You the man. Thanks