I made two false starts on trying for the "clean look" I envisioned. Finally figured out I envisioned it too late in the process, shoulda done so before the hatch was made so wide. In the end I'm making a H. Mahogany frame into which the polycarbonate will be bedded in my black Dow 795. I added cross members today slotten into the frame for added support of the panel.

I did use your marine board slider plan cut flat on the top for the flat panel assembly and it works great. The other guy who did work on this boat was sloppy (in MANY things) in putting the slide rails on and they are 1/4" narrower forward than aft. At least that works in my favor, if it had been the other way it would have been far worse. The hatch gets sloppy forward but the fact that the marine board slider has a top/bottom/outside edge, being machined from one piece, helps to compensate for the irregularity by capturing the brass rail.

If I'da had, to I would have put in a straight edge, flush bit routered the rails off from the narrow side and added onto the other side to make them straight but marine board sliders saved the day.

You can see the sliders in the pic attached to the bottom of the side framing. I glued it up with thickened epoxy and carbon filled for UV protection of the seams and also because the "theme" of this hatch is mahogany and black.. I reckon at this point to keep the hatch varnished since I won't have any other brightwork on the boat.