It worked. Considering it was a first step, and a first try, I'm somewhat impressed.
I ended up casting two takes. The first time, I mixed up too much plastic, and it ended up making a solid copy, just like the original. In the end, it'd make a good practice piece if I want to try carving it up.
The next attempt, I mixed a much smaller portion of the casting plastic, and then rolled it around inside the mold as it set up. I am using smooth-on Smooth-cast 305; and it gives me about 7 minutes pot-time and 30-minutes of cure. it took about 10 minutes to get glutinous enough that I didn't need to keep slushing it around. The result for this was better, and closer to what I wanted to get. It was non-uniform, and some of the places are thinner than I'd like, but that can be fixed with greenstuff.
So, now, I'm trying a third option. I took the mold, and put a layer of clay on the inside, and then poured a third layer of the mold material. Now, when it finishes setting up, it SHOULD leave a void that is about 1/8"ish thick. If I try something like this again, I may just do a single cast for the outside and then the internal plug. We'll see how it comes out....