Oh, ok. This would be the easiest way, but not necessarily the cheapest when you count in how much RTV you may have to use. I would use a plastic bowl, maybe an old tupperwear one. Ensure it is about 1-1.5 inches over the diamater you want to cast. Pour in your RTV mix to about 1 inch from the top. Them plop on your sphere. You should have some spillover of RTV so you know that the sphere is in contact with the RTV. A couple of the problems with this method, are you will need to use a lot of RTV depending on the curvature of your bowl, the other is that air bubbles rise, so you may not get a clean and smooth casting.
After the RTV sets, pull off your sphere and drop in your plaster.
There may be some other ideas out there, so I'll let others chime in as well, and you can chose the method that best suits you.
TazMan2000