Keep in mind that the plaster cast can't get wet otherwise it will fall apart. And putting preasure on it will make it brake. So fulling on the plaster will be difficult.
Tin foil will not make it water proof. So you have to work with plastic and buble wrap.
The more wool you use, the less shrinkage. I would guess it takes around a pound of wool, but that is just a wild guess. I do not know how big/ thick you want to felt it.
The edges are the least of your worries. Sure you can cut it afterwards. But not before you have done some fulling.
I think the biggest problem here is the fulling. Maybe make a threedimentional mold with newspapers that will fit under the cast so that it has at least the shape you can put some preasured rubbing on.( take away the cast then of course, otherwise it will brake) But also mind the shrinkage.
Hope this works out for you. If you are totally new to felting, then I would advise you to do a bit of felting over a shallow small bowl first. Then you get some idea what you are getting yourself in to. Have fun!

You could knit or crochet it to be a third larger and then felt it in your washer