Use a mold material that does not shrink when it cures (silicons
tend to shrink the least) either RTV or heat vulcanized. You can also
increase the metal of the cap to be molded with wax, making it as
thick as you want but you will have to use RTV mold materials.
Die making is another whole different subject and I have got to go
right now. If you want an idea or 2, drop me a line and I will answer