You can buy wax sheets for the job https://www.easycomposites.co.uk/sheetwax

Is wax paper thick enough? I'm thinking I will need something about 1mm thick...

Sheet Wax or layer of flash tape?

I want to make a mould of a part but i need to build thickness to it because the carbon fiber part that comes out of that mould will go over the original part. Is there a way to build uniform thickness along to ensure the molded part will fit over the original one?