Spirit gum or body tape should work. Resin is super lightweight when cured, so either one of those should do fine.
Was gonna come back and mention - MoldStar can get pretty spendy for even a small amount, so if you wanted to save a bit of money, you could use a push mold putty - you can find silicone mold putty on eBay in small, affordable amounts. Look for the yellow stuff rather than the purple, it's got a longer working time (the purple putty sets so fast, I hardly have time to push my models in) and is sold in smaller batches.
The only problem with push molds is that they're meant more for soaps or chocolate - I've found my resin cats will come out with a rough surface. But it can be wet-sanded smooth and then sprayed with clear gloss paint to get the surface shiny again. Also, be careful not to push your models in too deep, you can distort the width of your cast and then your pieces won't fit together as cleanly.
Just got my gold spraypaint today, it's beautiful! I can now highly recommend Krylon's Premium 18kt Gold spraypaint for gem settings. The finished products look like so: