Sorry, Sohum, I don't remember (it was 7 years ago) & we're not in a place where I want to get in the
water to check. Probably 3/8" or 1/2" (10-13mm).
If you go this
route, we used bolts as well as 5200, but you have to make the bolt-holes through the polycarbonate MUCH bigger than the bolts, to allow for expansion. Then put BIG washers on. Also, you should put little spacers in the corners so you know that you have at least 1/8" (3mm) or even more thickness of goop between the
hull & the polycarbonate, again, to allow for thermal expansion (or, I should say, differing rates of thermal expansion).