The aft cabin hatch is a bit tough because there is not much space above the hatch for the screen frame. The Oceanair frame is wider on the sides with the rollers so you want them vertical, and the narrower sides horizontal (at the top and bottom).
After reviewing the Oceanair site, they don't have one that will fit the aft cabin. The Lewmar 50 is the right size, but the wider frame (2.5") will have to be at the top and bottom and there isn't room for it there. If you go with a 20x20 it will hang down 5.5" below the bottom of the hatch.
They noted the Lewmar 60 would work for the main and forward cabins, and they recommended ordering them with seals. I pasted their email below.
Thank you for your inquiry. The seal is a rubber “gasket” seal that runs between the hatch and the headliner to block out any light that might finds its way through this space. We usually recommend using a gasket.
Also looking at your order form if the measurements you have provided are for the aperture (the opening for the hatch) the first hatch would be a SRS-60-(Color (B) for beige or (W) for white)-RP. This is a standard size Skyscreen for a Lewmar 60 hatch.
The second size is very close in size to our SRS-50-(Color (B) for beige or (W) for white)-RP. This is the standard size for a Lewmar 50 hatch.
We could always have a hatch screen made to order to the exact size, but that would cost significantly
If the standard sizes work for you please place an order with one of our following online dealers who will be more than happy to help you place an order.