It’s best to buy a cheap cast pan, fill it with chain or bolts, and through it on that. If you want to get fancy you can get a metal colander and fill it with ice and put it above that.
Baking tray with lava rocks is a better option, but the simplest option is just to bake in a dutch oven with a lid on. There's enough moisture in the dough to create required amount of steam. 20 min with lid, 20 min without gets the job done very nicely.
