The main issue is that there is no hatch available designed to be fully waterproof underwater for the size of hatch used on the Weta amas.
Also the Chinese-made floats with the seam down the side often don’t have a flat surface at the back of the float which makes it difficult to create a seal. You need to create to get a flat surface to mount the hatch onto and you may need to use some filler to achieve this.
Hatch suppliers have changed a few times but now I’d go with the Nairn hatches – if they don’t fit you may need to file the opening slightly. But you do need to remove any old sealant around the opening which can be laborious – I use Turpentine Substitute and a razor blade.
The old hatches used to be glued/sealed in without screws to hold them in place because the back of the float often wasn’t level. The 2015+ boats, without the seam down the side of the ama or the lip at the rear of the float, have a much better level surface at the back and now have screws around the hatch.
Tom’s mod doesn’t work on the Nairn hatches which lack the large seal area and use a recessed o-ring to create the seal. If you have a leak from the Nairn hatches, you can create a better seal by adding an additional 100mm o-ring.