RE: How many layers of 4.25 ounce glass is needed for a cowl
Also note that the shape of the glass, like curves ,bends, and humps increase the "stiffness of those immediate areas and help support flat sections. A flat sheet is not the best method to conclude that the glass is too pliable. Also note that your fire wall is really playing a big role in keeping the cowl shape.
As mentioned earlier, correct resins will also play a big role.
I use west systems resin in all my composite models. I have a BF 109 all composite running a fully cowl enclosed nitro engine/muffler and the resin has yet to soften from the heat generated. Even after flight with no air circulating.
You can even look into post curing with heat. this will add additional stiffness. Search online for methods that suite your needs.
Steve