Easy to make. I drilled into plaster with two different splayed (drill) bits. The first drill is the wide base. The the hole is continued with a thinner bit. The guide points make a coddle to catch the glaze.
Soft clay is pressed into this (first one should be thrown away as it will have plaster dust on it) and when it's leather hard it will easily fall out.
This glaze test is 100% iron bark (eucalyptus) ash from my studio fire place; fired to cone 10 in oxidation.