Thank you, @alexonit@twtxt.alessandrocutolo.it! It’s not sealed at all. If you were pouring in a liquid, it would run out on all four corners. It’s just folded over and carefully hammered shut as best as possible. 03 is a bit blurred, but you can see the tab from the right (the short side) tucking in on the left (the long side). The hem on top clamps it in place fairly decently.
I decided against blind rivets, because they leave ugly looking and sharp backsides, which can also interfer with the contents of the box. However, they would be an easy solution to make the corners more rigid and prevent any movement from the short sides.
Unfortunately, I can’t weld or solder, so that’s not an option. It would be the by far best solution. I wanna learn it one day, though.
Yes, Ken is a really great dude. He’s the reason I gave this a shot in the first place. :-)