Easy shaping material to use as blank/base/reference for silicone casting/molding
The terminology varies from &quot;industry&quot; to &quot;industry&quot; but the concepts are the same. I watched years ago a video teaching creation of molds from fiberglass cloth. The original was called a plug, the mold was created from the plug and the part was created from the mold.
In jewelry casting, the concept is the same, but the words are different enough that I was chastised online for using the wrong words. Their plug is called a pattern and the rest doesn't matter.
You haven't indicated what you'll be using for the casting material and that's not necessarily critical, as one can use casting resin or, with high temperature silicone, molten metal!
Your post states that one possible plug/pattern is not available in plastic. The material of the plug should not be a concern, as silicone sticks to nothing other than silicone. Mold release can be used for ease of separation, but is not always required. A metal original will release just as easily as a plastic, wood or clay version.
fixer1234's suggestion is certainly a valid one, if your objective is to create the plug/pattern, the shape of which will be that of the finished part. Sculpting clay that can be sanded and shaped by tooling after curing will provide for a finish limited only by your patience.
The same applies to 3D printed plugs/patterns. There will be layer lines, but they can be sanded smooth, or with ABS, vapor smoothed.
For that aspect, consider to locate a makerspace in your area. Our makerspace is closed due to quarantine, but our online resource is active and I've taken on commissions from people who need 3D prints made at non-commercial rates. You may find similar results in your area. With the internet, it doesn't have to be local, of course.
For wall thickness design considerations, use a multiple of 0.4 mm to ensure good results. The most common nozzle diameter is 0.4 mm. If you need something near a 1.0 mm wall, you'd want to design in a 1.2 mm wall. For 1.5 mm, design at 1.6 mm.