Gathering Beauty has a great article on making beautiful DIY stamped clay bowls. Air drying clay is a good medium for making special altar pieces and sacred receptacles. You can make them truly your own with whatever symbols you prefer, and you can add ingredients such as a pinch of soil from a shrine or energetic place.
In this article they use a bowl to shape the bowl, which makes it easier to control the size and thickness better. It’s a neat idea.
You can use any shape of bowl or receptacle, but bear the geometry in mind. If you’re using a square or oddly shaped tray or bowl, put a sheet of paper in it, and trim the paper until it fits well into it. Then use that as a pattern for cutting your clay.
Also, if the bowl is deep, there may be no way around having to smash the clay a little. To solve this problem, rub a tiny bit of vegetable oil (since this is an ingredient in many air dry clays and “play doughs”), and don’t stamp it until you’ve shaped the clay inside the bowl. Try to be gentle though, or the clay may stick to the bowl.
To make the bottom a bit more steady, you can also make a circle from a long roll of clay, or a disk to help the bowl sit well. To make it an incense burner and ash-catcher, put a chunk of clay in the middle of the inside of the bowl, and make a hole if a sufficient diameter to hold the sticks. You can make multiple holes to burn more than one stick at a time.
If you want it to be fire safe and resistant to humidity, you use the combination air dry and bakeable clay. You don’t want them melting or cracking.