These look pretty good.
Consider an oval shape. Circles often turn out to be ambiguous ellipses --difficult to align.
The two rings can be made by making a plywood form, wrapping it in plastic for mold release, making one ring, then wrapping that in plastic followed by making the second ring.

Seems to me that waterproofing would be more effective if the lid ring was on the outside.
If the rings were laminated, they'd keep their shape and the cross bar wouldn't be needed.

Wrap one ring over the mold, then the second ring over the first.

(A cool method might be to make bent wood Shaker Box rings )

: Here's the most "natural" looking hatches i've seen:
: http://dbynoe.blogspot.com/2009/05/skin-on-frame-hatches.html

: i cant really see why you couldnt do a solid substrate frame and a
: flush hatch, like a stripper...except for the leak issues, that
: i'm sure are common in that design... Something like a ring of
: plywood with a rabbeted edge, or built up that way, and a fabric
: covered plate.