[embee]

I have experimented with buzzers in the past on other bikes, the usual problem is getting something loud enough to hear at any speed at all without it being stupidly loud. Commercially available items sometimes have interrupts which stop it beeping when in neutral or with the clutch pulled in or brake on etc so you don't appear a complete twonk in town.

Simple way is a feed from each side indicator positive through a diode to the buzzer positive, the diodes stop the "other" indicator coming on when it shouldn't. Alternatively pick up a feed from the indicator warning lamp if it is easily accessible.