Dirk,
If you need a prototype I would recommend POP. We made a mold from it and it works great. We are currently working on a design for the next one. Unless you are doing major production POP molds work really well. Did you see the RTV mold that Mike Simpson had at Crappie Camp? He makes lots of baits with it. I would be concerned about the weld holding when you put hot plastic in the modified mold. Just my two cents.