A PVC mold would work, but you would probably get some nasty air bubbles trapped in there... I would suggest you make a Horizontal round mold like the ones in my mold tutorial. They would be ABSOLUTELY IDEAL for something like this. In fact, I am waiting on Ernie to post pictures of some material I cast for him that would have been alot harder in any mold except the Horizontal round!
PS... Nice pen Todd! I tried one of those once. I can iamgine how tough they are to cast, but i can attest to how hard they are to turn, (And I succesfully turned an Alt. Amber JG!) If you can turn that blank, YOU CAN TURN ANYTHING!