These are a larger version of our 15cm hatch cover - for a 20cm hatch hole

Hatch cover and rim with 4 flanges (the bits that go in the gaps to screw the hatch in place.
The O-ring is in the rim.

The hatch cover and rim are 260mm wide (for a 20cm diameter opening) - total with required if new fitting. You will need a flat surface./
There are 8 screw holes - 240mm diameter across between two screw hole centres on opposite sides.

Inner measurement 200mm (to edge of inner plastic but not the bayonets) - minimum hole size
Inner measurement 197mm on cover to outside edge of the part that inserts - see pic.