New water tank hatches - carbon fiber?

[scherzoja]

I am designing some water tank access hatches and want to make them stiff (no flexing) so when I tighten the bolts, the hatch will not flex the edges open.

I don't like the limited access that is offered by the round access hatches (Seabilt or Beckson, etc...), so I'll make my own. Two tanks are fiberglass, but not integral to hull, with three baffled compartments.

I plan on using epoxy and fiberglass (Probably 17 oz biax, no mat), then I came across the idea of adding carbon fiber.

I know I can make it stiff enough if it is thick enough,but I want to keep it as thin as possible to save space.

the hatch opening will be around 12" square ( +/- a few inches) with a stainless steel bar on the inside. When I tighten the bolt (or bolts), the bar and the lid sandwich the tank top and gasket to form a water tight seal.

I plan on making a female mold (I have 6 hatches to make) that incorporates 1/4" deep by 3/4" wide groves (half circle-ish) that will become the stiffeners an am considering using some carbon fiber in these stiffener grooves.

The attached picture shows large holes I cut for cleaning and repair. Final hold will be smaller, but around the 12" +/- 3" size.
Also attached is my preliminary drawing.

I've never used carbon fiber, any advice?

Thanks