I was going to make some asteroids, then I saw online and bought some asteroids that are produced by Gale Force 9 for use in our games here. I thought they were quite reasonably priced through one of our local games suppliers.
Some sit on the table or on flight stands, and are heavy enough that they are not easily pushed around the play area.
They have been well received by players in the games here.
It is quite different trying to manoeuvre through a realistic asteroid field than over and around tokens. I may try making some myself, now I have a better understanding of what they look like and what they're made of.