Edit: Saw that you already have a working solution...good job...I'm going to post my thoughts anway since I just struggled with the same question...
I agree with the above except for the fact if you want the install to be temporary / removable, use Marine Goop instead of epoxy. Epoxy would much more permaently bond to your hull. I just did the exact same thing with the install on my Mariner (the water well, make foam cutout, Marine Goop to hull, and fill with water before launching), and I decided to move and re-do it and the Marine Goop was easy to peel off the hull...just scraped it a bit with a screwdriver and it peeled right off. Note that it only peeled easily off the hull....it was pretty much permanently bonded to the closed cell foam cutout, and I had to make a new one of those.
Originally Posted by Wolverine93
Also...if you decide to go the putty route...electrician's putty, also called "Duct Seal", works a bit better than plumber's putty. You can find Duct Seal at Home Depot or Lowes in the electrican's aisle...it is sold in a 1 pound block wrapped in white plastic wrapper. The name brand sold at Home Depot is Gardner Bender Duct Seal, DS-110, costs about $2.
Wilderness Systems Tarpon 100, Native Watercraft Mariner 12.5