You can add more clay to the mixture , ferts or even dirt to thicken it. Or you can pour it into a cake dish as is and let the moisture dry out. You risk it being more brittle or a little mold this way. The mold is harmless though. Before it fully dries, you should score it to make pieces. Best to add more clay if possible. This method is a little more trial and error. Adding clay will make the amount of fertilizer less potent though so keep that in mind.
I think I'll use a jelly roll pan with wax paper, and with any luck maybe I can catch it when it's in a pliable state