are you using one of the powdered mold making RTVs?
those are cool, just weigh the powder, get the same weight in water,,,,mix together, mash into your Lego box with the part in the bottom,,,,,,,and you are done for a one piece mold, and just flip the first half and repeat for a two part mold,,,,,,,,use some Plaster if you are trying to save some money
takes the Yuck right out of casting a few parts,,,,,,,,or upgrading 2/3 of your Phantoms with the same quality pylons the other 1/3 came with
Rex
ps, I don't actually weigh it,,,,,,,I made a balance like we had in Chemistry sets in the sixties,,,,powder in a cup on one side, get water in the matching cup, until balanced,,,,,,,,pour one into the other, mix and use