I say go with the POP to get started. The RTV kits are 45-50 bucks for a small one and the plaster will let you get some inexpensive experience with building the form boxes and making the molds and not spend a day's wages o RTV. Let the plastic dry very well, then mix up some Elmer's glue and water and paint the cavity to seal the plastic. You may need to do a couple coats. Be sure to allow the sealer to dry hard too. I'd wait at least a week before trying to use the mold just to be certain there isn't a wet spot in the plaster you can't see. Plaster is very easy to work with.
Once you have a form or a box to be filled with plaster just set your bait, bottom side down, in the form with a spot or two of super glue on it to keep it stuck on the bottom and fill it up with plaster. Just be sure your plaster is wet enough to pour easily. Once filled, gently shake the mold box side to side to help release any air bubbles....a couple minutes of this is enough. Then just let it set up and dry until its hard enough to pop the plastic billet out of the form. I'd set the plaster on top of a water heater and maybe a furnace so that residual heat will hasten the drying time. When its hard, carefully pop the bait you used as a model out of the plaster and seal it up.
