More data to add. I have two cowls unfinished from Fibertech-n-More, the Cherokee cowl measures .021" thick in four places, the Chipmunk cowl measures .018" to .020" in four places. The unfinished cowl for the BUSA 33% Super Cub measures .030" to .034" thick, this was laid up by Steven at Aeroglass in Canada. Two cowls laid up by Fiberglass Specialties, 33% laser and 30% WH Cessna 150 measure .030" to .040" thick closer to .040" in most spots. And finally a blast from the past I have a Bill Effinger's W. E. Technical Services 33% Piper Tri Pace cowl laid up by Fiberglass Masters, you remember the blue colored resin, it measures between .050" to .070", rather heavy
I have a test piece laid up with 2 ounce and 4 ounce cloth, measures .010" thick, not thick enough for a cowl in my estimation, vert flimsy . 6 ounce E glass is just under 010" thick, I would shot for 10 to 12 ounces of cloth.