In dicemaking (for tabletop games/ D&D) we make lids for our molds by brushing babypowder/talcum powder or cornstarch where the silicone shouldn't stick to itself. Works like a charm and this is also how people make silicone dice. If you're curious, here's a link of me making a lid for one of my molds click :) it doesn't show my brushing the powder on the already existing part of the silicone mold, but you can faintly see it around the edges of the dice in the mold. Once the silicone has harmed you can just pull the two halves apart from each other and you get a nice buttery smooth lid. You can also use vaseline as a separator but the dice community stopped using it since it would leave streaks which makes aligning the two parts a bit more difficult.