Here's a way to make asteroid belts that are quick to deploy on the table.

Take either stiff felt, wood or plastic and cut the material into shapes that resembles connected hexes that match your star mat. I usualy print out a sheet of hex paper and cut it up along the lines to make "clusters" and lines of hexes.

Paint these shapes black or in a star field fashion.

Next create several asteroids in what ever fashion you like. I use half inch styrofoam balls and paint them gray. Regardless, leave a flat bottom for gluing.

Then glue each asteroid into each individual "hex" that makes up the shape. i.e. if you cut out a shape that look like five hexes in a row, glue on five asteroids.

Now you have several asteroid "clusters" that you place out on the map quickly. You'll discover that the hex shapes allow you to match clusters up like a puzzle if you want an area thick with debris.

For me, the shapes inspired me to creatively place them in fun formations. The first game we used them, it inspired a "badger in the hole" scenario where one fleet hid in a cluster of asteroids and the other fleet had a limited time to dig them out. Just like the movies, we both sent our fighters out in to do the dirty work before the ships crept towards each other to finish the job.

Too cool.