FWIW - a few observations from my E27 hatchway.
My hatch is wood that has had some topcoat put over it. To be honest it looks like automotive undercoat paint or bedliner. I'd love to strip it but I strongly suspect some cheap wood is hiding underneath.
The frame appears to be standard rectangular wood slats on the sides. The front and back pieces are flat on the bottom, with a curve cut into the top.
The bottom (inside) of the companionway wood is exposed, and you can plainly see the parallel router marks that were made into the wood to add flexibility. It would seem to me that another bottom piece should be there, covering the routed piece and adding strength. I've not tried tap dancing on it but it seems sturdy enough to support your weight when reefing or when transitioning from the top to the cockpit.
It would appear that the wood was nailed to one side, then making use of the added flexibility, was "rolled" over the frame and nailed to the other side and front/back pieces. I can post pics if you wish. Honestly, it looks pretty basic. If I were to rebuild, I think I'd go with a really nice looking wood frame - something that looked good - and liberally varnish it to keep it looking that way.
hth,
- Jerry