Ed
I don't know if this would work. Just a idea.
What if you took a piece if two inch thick pink foam board 3 or 4 plus feet at a time 7 or 8 inches high and dug out one side as deep as possible along a long piece. Different depths and angles. Try and make your own mold.
Then fill the different depths flush with plaster ( like dry wall mud)?
Just sitting here thinking??? I don't know if I'm explaining that well.
Larry