You could try D&R Marine and see if they have something.
I see a hatch for an O22, but not for an O23…
Otherwise, you could measure the size and have a local glass/plastic shop make one out of Plexiglass. You would probably need to drill it for a stop in the back and a handle on the front edge. But it would not be too hard.
On my O’Day 322, a PO made a rounded slider. Keeps the water from pooling on a flat piece of plexiglass. But it is heavy.
This is from yesterday…yes, that is snow on the companionway
Greg