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.
Regarding safety, just be careful not to burn yourself