If the clay is water based i wouldnt worry about a release agent or similar as its only a small peice. and you can carefully rinse out any clay residue after.
If you make a DAM or wall around the outside it will help stop the plaster running away if you make the plaster alittle runny, you should make a small batch 1st and use a old brush to lightly brush it into the details to make sure they are all picked up properly, then a thicker batch on top.
I guess you have probably done some youtubing or reading etc, so i am probably repeating what you already know.
