Despite the OP's reservations I think that prop guards are the way to go. They are a collective measure that requires no user input & are more or less idiot proof. They also serve multiple purposes in that they protect props AND people. A buzzer is easily ignored in the heat of the moment, especially if it's going off all the time, the users will become deaf to it. The technology will require some user input, & at the price point the OP is looking at, it ain't going to be top end quality, especially the NASA stuff.
Just my 2 penneth, IMHO yada yada....
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5