Easy shaping material to use as blank/base/reference for silicone casting/molding First of all forgive me for not using the correct term for this. I know there is a proper name for the very first mold that is used to create casting jackets, but I cannot remember what it is. Hopefully my description will be sufficient, or maybe someone can in due time post an edit with the proper term in order to improve this question, which I would be very much appreciated.
I am trying to create 2 very basic shapes and I am planning on using silicone casting to make them, but I need a reference object first in order to create those casts. I am trying to find out what would be an easy to obtain, and easy to shape material that I can use at home to create these two shapes by hand, with minimal tools required (a simple file, chisel, easy and basic tools that can be found in general hardware stores):

These are examples of the two objects I want to create. They are about 1mm or 2mm, I'll be playing with those dimensions after. One is a simple flat square piece with rounded corners and the other is a squared short tube with no bottom (in the photo there is a bottom, but for what I want there won't be one) and rounded corners as well.
What kind of material can I easily obtain in order for me to shape them into these &quot;raw&quot; first examples in order to create a silicone cast?
To make it clear, I do not want to find a material that can already be used to create the objects out of the silicone casts, I just want to find a material that can be easy to find and shape in order be used to create those silicone casts/molds first.
EDIT: Information requested.
The 1mm or 2mm are the thickness of the walls of the hollow shapes and the thickness of the flat pieces. This does not need a lot of precision, but shouldn't deviate too much, about 0.5mm or it might be trickier for what I need. It will not require that much precision fitting, these will be used to build a small box, like those typical ring boxes from jewelry stores, that is less than 4cm all around and they will be lined, so the surface finishing is also not necessary to be perfect, but if it can be sanded it will be even better.
I already looked for rectangular tubing, but for these dimensions where I live none exists. It's either too big, too small or not available in plastic.
I also already looked into 3d printing, but the options I have available are closed down because of covid, and the ones that are open are too expensive due to min-order purchase.
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.