I've never done exactly this but I'd approach it the same way I'd
approach a bead -die set for a plain bead half. That is to fabricate
the male punch in steel, then rough out a depression in a nylon block
(with a round bur), then heat the steel tool up and melt it's shape
into the nylon (female die ).
The ones sold at places like IJS, Thunderbird, and probably Rio
Grande, also start with a male steel punch, but they use steel for
the female too, and heat it up till it's soft, then punch the punch
into the die block to transfer the shape. That's the old -fashioned
way, and I couldn't say for sure that they aren't mass produced by
more modern methods. Probably by graphite electrode EDM machine,
where the female die is burned out by electrical discharge using a
carbon rod with the desired shape on it's end.
If you want to make your own, do what I would do: take a round dap,
or even just stick a piece of bolt in a drill and file the end round
( a soft bolt will be easier to work than a hardeneed dap). Then you
file the flutes however you want them, into the rounded bolt end, and
there you have it. When you use that to melt the shape into the nylon
(use the white stuff, not the black stuff that has molybdenum
sulphide (?) in it ), you need to do that carefully, and in stages.
You don't want to do it all at once, or get the tool and nylon too
hot. When you dap your fluted beads, you need to dap them round
first, then anneal them. Do all that right and you should be making
some kickass beads.
Dar Shelton (or you could make a plastic steel female die, using the
male punch as a model)