I have worried about this guaranteed failure on my H22 every since I got it. I presently have a 2 inch crack that will eventually look just like yours.
I have seriously considered looking at replacement 'windows' for skylights.. I think there might be a fit there, and would not be surprised at all if that was not the original source.
A higher end building supply place, or the internet, might supply an answer. The arch in the companionway is going to severely limit how well a flat piece works. If you use a flat piece, I think you're going to end up building a frame that has the arch built into it and is 1.5 or so inches 'tall' to get over the entire arched portion of the hatch.