In my college comp-sci networking class, one of our projects was to figure out a system for sending messages visually between two parties spaced 100 yards (91 meters) apart as quickly as possible.

Each team of four came up with their own signaling method, trying to balance how many bits were transmitted with each symbol with how quickly we could encode/decode the message.

Binoculars were not allowed. We were outside, so we didn't know how windy it would be.

I seem to recall on my team, we were sending two bits per symbol. I think it was just a large cardboard box, we colored each of the four flaps visually distinct, then unfolded one of the flaps for each symbol.

I don't recall a single team thinking to re-purpose naval flags.

Duh.