I’ve been able to play around with a Nikon D810 and see how it might fit in my Aquatica AD800 Housing.
In general, it seems to fit physically, but will need modifications or changes to several control parts to reach some shifted camera button locations.
For example, the saddle works perfectly, but is impeded to fully lock into place somehow, perhaps by the new shape of the grip.(???) Therefore, I could not close the back without something being modified. The lens axis and viewfinder axis seem to be correct, and the shutter button seems to be in the right spot. But the front command dial mechanism would need a change in location upward a bit, as it does not engage at all.
The good news is that perhaps with a refit of some parts, I might be able to re-work an existing AD800 shell to about 80~90% functionality. There are some controls that just can't work. In conversation with the folks at Aquatica, they feel that a "retro kit" of sorts just wouldn't be practical, and if I read their tea leaves correctly, it means an AD810 would be in their future (by November at DEMA???) Personally, I’ll probably just stick with an 800 setup and the 810 will serve as a topside camera (as others will, it seems). For now.