That one is only 19 ga. thickness. It’s meant to be attached to a fiberglass roof.
A cookie sheet, pizza pan, or pie plate like this will work:
10 Inch Round Pie Pan for Baking, Aluminum Commercial Grade Pie Plate by Tezzorio https://a.co/d/2fHKb9h
Don’t let us convince you that the size is too critical. These antennas are typically fastened to a metal roof of indeterminate size. Also you’re on the water. That’s just perfect in terms of terrain.
I’ve never put an antenna on a boat, and I don’t know what your boat is like or what it has for mounting, but do whatever is easiest.
Edit: I told I’d never put an antenna on a boat. It appears that for marine applications the body of water serves as the ground plane. I really don’t know how you take advantage of that.