You can use something like RTV, but wouldn't recommend it. They do make hardening catalysts for RTV. I'm like the rest though....doing molding/casting/etc....the time & money it takes to use good materials (including plaster, resin, plasteline clay, etc), you'll probably be over-cost of just buying them. I'd really only approach this route if you were going to mass-produce them. After the 1st one is made, you can do a bunch more. For just yourself, I don't think it'd be cost-effective.
Good luck!