Dori,
The dental stone & plaster we use isn't very good because it'll break down in water no matter how hard it is/gets. (I work in a lab)
You could try the yellow stone, but mix it extremely dry so it's at the hardest stage it can get. No, not the way you mix to pour a model, make it much MUCH dryer of a mix. You'll want to see it dang there hardening up as you start mixing. Make the plug and test the strength of it after a week. You might be able to simply snap it in 2 with your fingers.
I'll run a test myself as well using more exact mesurements of 2 brands of the yellow stone.
Red,blue,green,white dye-stone is definately a no-no. As well the plaster is just too weak to even try.