Some good questions. I’ll try to answer a few.
But I’ll start by asking one—do you currently use 3dCAD software? If not, to do 3d printing, you’re going to need to come up with the files. There are many options, including zero cost online apps, and this would lead to a separate thread.
Be aware that the most common hobbyist 3d printers are extruders, using plastic filament that is applied in layers, and so you do not get a finished, truly clean surface. There are also resin printers available, especially with small envelope (or platform) sizes, that are both capable of much cleaner prints, and more problematic to get to the point where you are just confidently popping out parts. Note that even resin printers will need some cleanup, but much less than filament (extrusion) printers.
There are also print shops, online, and if you’re in a bigger city, locally, that can print parts for you. Something knob sized would be around $10 in a polyjet print, which would just need a very light sanding.
Casting is very problematic, for a hobbyist. If you’re willing to drill out the holes for the knob shaft and screw as secondary operations, you could do open faced molds, which are FAR more easy to have success with. But otherwise, you need a two part mold, and you’ll still have to drill and tap the screw hole, since you can’t have through holes in soft tooling (what the mold gets called). To do a decent job, you need a vacuum pump and vacuum chamber, to de-gas both the mold material and the casting resin. Unless you use a silicone for the mold, you will need to deal with release agents, which can be hard to use without leaving an impression on the cast parts. In my shop I always needed to look at the clean up time on prints v. The number of parts I needed, and figure out where the break even point was between multiple printed (or machined, etc.) parts and making a master and casting from that. Your idea of making multiple masters is good. If you’re interested in casting, look at artstuff.com—Douglas and Sturgess have been my casting material supplier for over 40 years, and their online catalog is almost a textbook.
Another issue will be where the set screw taps in. Most filament resins are too soft to really hold threads well; even cheaper mass produced knobs ( which are generally made from harder plastics) use either brass or steel inserts where the set screw (and often, knob shaft) go, for longevity. You could always design a slot to hold a nut or a threaded insert in your design—this would be a clever way to save a bit of work also.
I don’t mean to discourage you, I’m all for DIY. But approach the casting, at least, with a sense of really needing to go through a number of less than workable results until you get the process nailed down a bit.
