I have done what you are talking about with one difference. I used Alumilite silicone mold making material to make the actual mold. The parts come out really easily doing it that way.
I'd been casting alumilite in 1.5" pvc tubes for the barrel and 1.25" for the tone board. So I just went ahead and cast two blanks, then decided what the shape to turn those too in order to make the silicone mold "positives".
After I cut the blanks to shape (with the hole in the barrel) I found two plastic pipes the right size to fit over those two new shapes that gave me enough wall thickness. I used those for the making of the mold with the silicone resin with the "positive" call part glued down to a piece of Lexan and the pipes glued down over those "positives".
They came out good, I've made a number of calls from them, and they use less then half the resin compared to before. The one thing I would have done differently is to give a little extra height for each mold.
Happy turning
Tom