Man that stings, a maple board is not cheap and 4 hours is nothing in dry time. I did almost the same thing last winter with a box fan on a walnut cutting board I'd been working on for a week. The air moving across just one side pulls the moisture out of that face way faster than the other, so it cups toward the dry side. Next time try pointing the fan at the wall so it bounces, or just set it farther back so you get a light breeze instead of a jet. Sorry about your board, hope it's something you can plane flat again.

Idk about spots, but hanging stuff up top means you gotta check what the ceiling joists can hold first.

Tbh the "4mm corrugated" thing tracks with what I found messing around with mockups. I used to grab the thin cereal box stuff too and it always did the same thing, folded right at the crease the first time anything touched it. What actually worked for me was doubling up two layers with the grain running opposite ways and gluing them with a thin spread of wood glue, then letting it dry under a stack of books overnight. That made a huge difference because the stiffness comes from the two skins being separated, not from the paper being thick. For phone stand angles I just do the double layer thing now and it holds up fine even with my kid bumping the table. The cereal box stuff is still fine for pure eyeballing before you commit, just don't expect it to survive actual handling.