I’d propose a different construction, though it’s a bit of a heretic idea to make holes in the hull…
Use longer aluminum crossbars and make them go through the whole hull, using plastic boat thru-fittings:
https://www.amazon.co.uk/through-hull-boat-fittings/s?k=through+hull+boat+fittings
I’d imagine if you cut off the unnecessary length in the hull fitting and choose tight fitting tubing size, it would be pretty airtight. Perhaps with some glue between fitting and hull fabric. I don’t promise it will work, but if it does, that would make quite a solid connection.
The 450g nylon from Extremtextil is much stronger than their 275g or even 370g versions. Bigger difference than the numbers indicate.