For what it's worth I sailed a Shannon in the Perfect storm and we got beat to hell. The boat had acrylic hatches that I doubt were 1/2" and had no failures.
Our current boat has cruised 30k plus miles with Goiot hatches. One is 24 X 24 with no support bars and 3/8" casat acrylic. Zero issues. I just swapped the acrylic this season after 30 years of service with zero leaks and zero failures despite many, many, many off shore miles.
Someone at my club threw out a 21 inch square Goiot hatch in good condition excepting that the acrylic had a crack in it. I scooped it, and will replace the acrylic and repurpose the hatch to go into my forward collision bulkhead (above the waterline) as a way to get into the forepeak without going on deck from the saloon, plus a way to air out that area.
The collision bulkhead has limber holes that would normally remain plugged on passage with rubber stoppers.
Yes, it will look a trifle odd to have a clear deck hatch in the "wall" of the saloon, but as far as I'm concerned, it will solve a number of problems, will save me $1,500-$2,000 from the sort of "submarine hatch" one might normally find, will get air and light into a dark part of the boat, and will allow us to carry long items in the main salon that might be torn off the deck in bad weather, like whisker poles or our Portabote.
I intend on having this hatch close to the "ceiling" of the saloon, and if I've got water in the forepeak within 22 inches of the underside of the deck, having a hatch there is the last of my worries.