How to make a tile

Total guess, but how about a polyethylene container intended for food storage. I'm pretty sure it would cleanly release the tile.

ohh great idea Roger...I could cut it down and pour to thickness....

Just pick something that doesn't have tapered sides.

You could use a strip of shiny cardboard as wide as the thickness of your tile. Fold it into a square and tape it to a flat surface. (More shiny cardboard)
If it's not the shiny kind, some of the plaster will soak into the cardboard and won't release.

aah, thank you...I wondered how it was going to release...if not shiny cardboard, maybe try some silicone material.

Plastic, glass, metal... all will release easily from plaster.