Sewing machine, epoxy or some other type of adhesive, staples might work in some cases and if you add a little something on the side that gets punched through so the staple won’t pull through, a tiny bit if thin cardboard maybe, it will hold.
I need to make some doors or flaps too right now but I am tired with fucking around with flaps and light, so I am trying to think this one out. Let me know if you come up with anything. I have more pics of what I am trying to seal up on the last page of the grow in my sig.