You can use anything as a mold if it’s made of plastic. Metal molds will work, but you will need to
temper the chocolate, which shrinks the chocolate slightly when cooled so the bars will slip out of the metal molds.
It’s difficult to pinpoint exactly how much chocolate you’ll need. But as an example, for the 4 bars I made, I used about 8 ounces (230g). But the good news is that you can just melt what you think you might need, then any leftover chocolate can be poured on a sheet of parchment paper or plastic wrap and left to cool to be reused for another baking project.