Cable ties. Punch four holes in a square pattern in the flap and use them as a template to mark and drill matching holes in the guard. Then use two cable ties to hold the two bits together.
Buy a rivet gun and it usually comes with a few rivets of various sizes to get you started. It's the sort of tool that you'll find loads of uses for once you have it.
How are people attatching flappys for their mudguards?
Bovril? super glue? good faith? hammer and nail?