Yup, that IS a tough size to match...
"XR150
I.D: 16" x 16" (40.6 x 40.6 cm)
O.D: 181/2" x 18 1/2" (47 x 47 cm)
Acrylic: 17" x 17" (43.2 x 43.2 cm)
Weight: 11 lbs. (5 kg)"
This model from the
http://www.atkinshoyle.com/xr.html
site looks close. If the cutout is only a half inch more, that should be do-able, maybe.
(The folks at PlasTeak do not show in their pictures if they are making a framed hatch. Making a replacement for a plastic panel or acrylic panel hatch is a lot simpler -- just a routed channel for the gasket and no frame. I cannot tell from the site information if that is what they are doing, though. But, if so,they should have plenty of business, what with this style of hatch used on a slew of aging low-end powerboats and sailboats from the 60's and 70's.)
Adding more search information: Vetus now has a model "Genius 4040" that has a cutout of about 14 3/8" square, from what I can read on their web site
http://www.vetus.com/frame-cat.htm
In this situation, it is often easier to shrink an opening, since the housetop molding had a raised flange that typically faired into the adjacent nonskid. Well, it was just a thought... and I am reasonably comfortable with light-duty mods like this...
Loren in Portland