I remember seeing someones RIB with a large round hatches in the transom boxes. These lids where hinged and also hid the screw holes. I'm pretty sure it was a Vipermax I saw them on, and I've been searching the theads and can't find the info on these.
Also looking for the smallest retangular locking hatch I can find, also with hidden screw holes.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
I am on a similar quest. Looking for hatchs, OD very approx 300 x 300 preferably with a non hinged lid, ie removable lid. Doesn't help you I know, but...............
Will keep on the look out for the hatches you are looking for as I search