Honestly, get a fire ring from tractor supply or any sealed fireplace. As long as the burn stays on the top you’re good to go. I’ve made plenty with a regular outdoor fireplace. When my kiln burns through this is what I’ll use. You can even make the same shape with soil. https://www.tractorsupply.com/tsc/product/tarter-farm-and-ranch-equipment-3-ft-fire-ring