I covered the old fiberglass companionway hatch cover on my '68 E23 with teak. Using the old hatch for the frame simplified the process for me. The old hatch cover had three fiberglass ribs protruding on the top side. To keep a lower profile cover I cut the ribs off. Next I glued, screwed and biscuited 3/4" x 3" teak boards to the top. Then made front, back and two side pieces to cover the frame sides. I also made the hatch to deck tolerances a little tighter (smaller gaps) while I was at it. Took a few days but was a fun project. Here is a photo of the finished hatch cover and new drop-in boards.
Mark