Anything for the shaving head end with the female threading you need can be sunk into the handle. I have a handle that cooncatbob of Bob's Razorworks made. Sadly he's no longer with us. He died two or three years ago.
Check out his work though. His old website was still up the last I checked it. Anyway. If you can finish the handle at that end thin enough to fit a vintage Gillette Tech head it would fit most other kinds of three piece razor heads neatly. One I have is too thick for the New but fits the Old Type as that's what I had it made with, (vintage head replated also)
Bob in a way kind of started the boutique or "customized" gear, safety razors, and wet shaving as a niche hobby for me personally.
On the butt end of his handles he custom made knobs out of brass he put threaded rod stock into and plated, if ordered that way. Some of his top ends were extraordinary when made of metal. He turned single and double rings, and a variety of unique fluted razor head handle ends. Usually matched or complimentary of the bottom end. He did custom knobs for Progresses too.
One of the things he did that was especially interesting is, on his three piece handles he would either make the head end completely from scratch or re-use ferules from the original vintage piece if he was customizing a new handle for a vintage replated head. Mostly vintage Gillettes. I don't believe he ever made an actual razor head from scratch. I know he didn't care for the Indian or Chinese kit piece heads and was constantly scouring eBay for vintage Gillettes to replate for his handles. He even for a time used Gillette Knacks and Slim Twists for his TTO razors. I'm pretty sure he just epoxied the heads of those into his handles. There's one in the pictures I linked below.
What he did best was make his pieces modular at the end of the day. He used threaded rod pieces that matched top and bottom. So he did mixed mediums. He made wood and brass mostly. Handle bodies as well as butt end pieces.
Sort of like the Razorock Mamba has recently. A knob. The very end is not permanently attached. So you can mix and match. Balance not to your preference? Try a wooden end knob, a metal one, none at all...
I dug the concept of having a handle nearly custom balanced and tuned like say, really expensive golf clubs.
The fittings he used were probably press fit. I doubt he custom made most of them. The thread size is whatever the metric one is that is most compatible. I can't think of it off the top of my head at the moment exactly what the size is.
The knob ends he made look similar to tiny sized drawer pulls to me. I actually bought the smallest drawer pulls my local hardware stores had to finish the end of a couple of custom handles I made for myself out of wood and bits and pieces of vintage Gillettes.
Should be a similar situation with acrylic as it is with wood to track down the female threaded hardware. Just leave both ends the same female size as you'd use to accept the razor head. Male thread the tops and bottoms and then throw a little variety into the market. Then as your store takes off, your customers can mix and match your work to get their own perfect balance, length, look, whatever they want.
I'd think basically if you buy a nice tap and some threaded rod stock you can make end knobs out of just about anything.
Here's some of Bob's stuff that I have. A Google search would probably yield you better pictures and info about his work.
https://photos.app.goo.gl/ozCNPi6SRLE6NeaP9
Here's his old website.
https://sites.google.com/site/bobsrazorworks/
Some of it's kind of broken now.
I think unless you're going all out, selling razors with generic or "off the shelf" heads will hurt you. Might as well just sell the handles themselves. Most guys have a collection of different brand three piece razor heads already.
Have a look at Toby's work of Triad razors too. His work is all metal I believe, but he often made modular pieces as well.
https://www.google.com/search?q=Triad+ra...e&ie=UTF-8
What I take it you're talking about is making some turned handles as a value added compliment to a wider variety of products. Is that right?
What other kinds of products might you be considering focusing on? Or were you just referring to handle production when you said mash of vintage and modern?