Something "made fast" is perfectly reasonable usage for tying it in place.

Yep. And something "made fast" is also perfectly reasonable usage for vastly upgrading its speed. "Fast" can absolutely be a contronym.

I think two-word contranyms are more concrete, like "fight together".

Oversight is a less ambiguous example of a single-word contranym.

I've heard "fast" as a contranym to "moving quickly," referenced as "to fast" or "fasting." ie: not eating at all.

Edited to add: Sticking with the context of food, "fasting from food" contradicts someone being a "fast eater."

"Fasten your seatbelt" doesn't usually mean "twirl it wildly above your head".