I want the text to appear to be floating, so having it mounted to glass or plexi wouldn't be ideal.
How do you see yourself mounting the letters ? Couple of pictures of what you have in mind would be helpful, either something along the same lines or even a hand drawn sketch
What sort of size are we talking about ? Guessing not to big lol
You will have to mount the letters some how, possibly on extending rods, either one per letter or less if letters are joined together or you use a script font where the letters are joined together naturally.
Don't rule out using acrylic to mount the letters completely, a solid acrylic shape may not give the look you want but a lot of that is down to viewing distance, the further away from it you are the less noticeable the acrylic will be so the more the letters appear to be floating. Another way using clear acrylic is to use a thin acrylic rail or in this case an acrylic ring to mount the letters to.
You then just need to figure out how to mount the whole thing to your fixed shaft which could be done with 2 or 3 rods or you could machine the mounting arrangement along with the acrylic ring all as one part. Another way might be to use small stand off locators to fix the ring of letters to a separate piece of material fixed to the static part of the shaft.
Difficult to give more detailed advice without knowing more which you may not want to divulge if you are looking to manufacture these
A lot will depend on just how much time you want to put into making them & what sort of price you are prepared to pay each.