Well heres the final piece (well the first one I've cast, theres a bit more refinement needed on the mold)
The original piece was hollow to save money on printing costs, but when cast I thought it might be too weak to hold a belt in place so I filled in the holes with a plastacine-type-thing I had laying about. The belt loop on the back had to be snipped off as well, it wasnt castable as it is in the design. After snipping that off and creating two new loop holders out of clay I started the mold making..
I made a silicone mold using a rubber called RTV101, but first coating the piece in many layers of spray-primer. I think there are threads on here about that, so check them for more information
Once the mold was cured I had to add some air-holes and scrape out all of the plastacine that hadnt released itself from the rubber (the mold release stuff didnt seem to work here..).
After a few failed casts I managed to get one that worked
I polished that with a dremel polishing thing and sanded it to give it a more roughened look (it looked too blingy when shiny). I drilled some holes in the belt loop on the back and added a piece of bent wire to hold it in place.
Its a little heavy but works perfectly, I may try to cast a hollow one and see how it holds up.
Next step: tidying the back of the buckle up and listing them on Etsy. Then hopefully I can start making Tubgirl and 2Girl1Cup ones...