This is a time honored technique. The more you do it, the better you get at it. Once the plaster in the bowl gets thicker you can plop a final 1 to 3 inch thick layer onto the sculpture. You definetly want a minimum of 1 inch thick plaster over the entrire area. A side shot of the plaster half.
Lets grease up the back of the sculpture and pour a plaster cast over it.
You'll want to grease up the area that the two plaster halves will connect at also. Grease that up good or it will be very hard to get the two halves apart. A coat of Clear Acrylic paint wouldn't hurt here either. The back half plastered over. Notice the tight connection area between the two halves.