Here is the way now i am doing the bases for the Fossil series. first a plaster base -that's simple, I just pour plaster in a vague form done out of plasticard that i sand afterwards.
Then when cleaning my plaster dish, I put one the water with the plaster wash on some plastic surface that I leave to dry.
Then I break it with the hands and "glue" it on the plaster base using some ultra diluted plaster.
I just have to trim it and paint now.