Do you need to do this for mass production, or for a one off?
Molding silicone parts can be done fairly easily and fairly low cost. One off parts can be molded in a DIY fashion but it depends how many pieces you are trying to get out of it.
If you are just trying to do a one off, RTV or caulk might be able to do the trick if you don’t need to ever disassemble it.
Posting your part + your intended quantities and budget is helpful. Sometimes doing it the right way ends up being cheaper in the long run than science projects.