I have the Atkins & Hoyle hatches on my 1980. The forward overhead is about 24x24 inches, I recall. The acrylic piece is about 21.5 inches square (but not quite square). I could check that at the boat in the next day or so.
The opening is about the same dimensions as the acrylic, I think. Maybe slightly smaller.
The opening is about the same dimensions as the acrylic, I think. Maybe slightly smaller.