I made this part for my 2003 Crestliner 1850 Sportfish, but I'm sure it will work for thousands of other boats out there as well.  It should fit a hole with a diameter of 1.8" to 2.25".  This should fit any wall thickness between 2.25" and 2.75".  

There are several plates in the print profile.  I suggest printing with the outer faces AWAY from the build plate as this gives a better finish to the rounded edges.  Also notice that the flapper plate is in TPU which I believe is essential to this part which must flex to let water out, but stop waves from pushing water into the boat.  I also included a couple of tools so that you can comfortably tighten this by hand and avoid using any channel locks or other tools that will leave permanent marks.  Any small stainless steel screw should work for screwing the flap in.  The small part with the pins in it goes through the TPU flap and then inserts into the holes in the outer fitting.  I also used bamb PLA/PETG support interface in this print.  I suggest using PET-G or other UV resistant materials due to the outdoor use of this product.

You can print without support interface as well.  I also suggest printing without thread supports by changing the threshold angel to 29 degrees or less.  I will upload a better print profile when I have time to print it again.