So it took me a while to get back here...
I propped up the kayak on some 2x4s at each end, taking all of the pressure of the center of the hull. The box would still not fit. I do believe the issue was that my hatch hole is about 1/8" off center. Yes, it is just 1/8 of an inch(!) right of the center line, and this is enough to make the drop in box unusable.
This guy had the same problem:
http://www.saltwaterkayakfisherman.com/ ... hatch.htmlI used his bucket fix, although I only did it on one side, and fixed the problem.
We installed the hatch on my wife's 2017 Outback, but we spent a lot of time lining up the cut to the center line before proceeding, and her bucket fits just fine. Because of the shape of the hull, there is not a precise way to easily mark a center line, so it is easy to see why so many people run into the same issue I did, while so many people don't.
We have been out in our kayaks at least once a week since we bought them, and love them. But it is really frustrating that you spend $2500 on a premium kayak and it comes with a flimsy main hatch that is hard to use, hard to seal, and dumps any water sitting on the lid right into your "dry" tray and the hull. The Outback should come with the square hatch from the factory. The interior screw-backing ring on both of the round hatches we removed were so warped they looked like lasagna noodles, and these were brand new kayaks from the dealer. Neither one of the hatches would close all the way. The square hatch is so much beefier that it forces the whole footwell area of the hull to be perfectly flat, while the flimsy circle hatch conforms to the flexibility of the hull. I won't even get into the strangely-profiled o-ring they install on the circle hatches because that has been discussed to death.
Now that I'm done complaining, I will admit that I should have gone with my instinct when I first cut the hatch hole and not trusted the circle hatch to be centered perfectly. I think it was a reasonable assumption, but I know better than to trust quality control of any company these days.