Minimum of 4 layers of 4 oz.

Better 6.

The distance expanse is not large at all and is reinforced by the hatches.

I make glass seats for my kayaks and I usually use all the scraps around which usually amounts to a minimum of 6 layers of 4 oz and sometimes up to 8 in stress areas. It's important to remember that epoxy/glass will harden over night but won't be rock hard and stiff like it will get in a week. One time I made a seat and was cursing myself for not using enough glass. I was going to add more glass and didn't get to it for a week and I was amazed at how stiff the seat was at that time. It never needed anything more.