The full costume here. Traditionally, batman has a gold belt, but I thought it would be better to make the belt black to fit in. To start I took a strip of black fake leather and cut two holes into either end. These are wide enough to fit a strip of Velcro, and I threaded that through all four holes to make a clasp.
For a 'belt buckle' I made another batarang and then put a strip of one-sided sticky velcro onto it. It can then stick onto the front velcro clasp of the belt.
For the sides of the belt I took strips of black plastic and cut them into rough V shapes and then attached them with Goop.
Or if you want to just buy one: