3D-printed Gaming

BTW, in FreeCAD, you start a project, then you use File > Import to bring in the STL. It is better if you can find a STEP file because these files contain the actual geometry instead of a mesh, but I didn't see one in a quick search. The real difference is due to π and how computers need to round infinite numbers. A mesh is ultimately removing the required rounding uncertainty that π causes... If you care about the details...

Once you have the STL in FreeCAD, there is a pull down menu for workbenches. Find the Part workbench. This WB is like Tinkercad or like playing with wooden building blocks as a kid. Once you are in the Part WB, highlight the STL file, then use the pull down top menu bar labeled "Part" and select the option to convert a mesh to a part.

This does not recreate π, so it does not recreate everything like the original. It is still a mess of triangles in this case, but it is a solid part at this point and not a container like a mesh. It still won't be perfect and the usefulness overall is debatable.

I've learned the hard way. I will only use a converted file like this for the most simple one or two step operations. This is what I was looking at doing for you if it seemed doable, but I don't think that is the case. My rule is to recreate the geometry properly in a Part Design Body in FreeCAD using sketch based design techniques. I don't actually use the converted part for anything more than a background visual guide.

The way I would remake this is complex for a beginner. The main challenge is the whistle itself. I would need a lot of trial and error of actually printing the thing and tuning it. I couldn't make something that works otherwise. I don't think anyone else can do this either.

I don't mind helping you learn how to make something like this, but there are a lot of little concepts you need to grasp with CAD that can be challenging to wrap your head around all at once. I'm good at explaining abstract concepts, but your level of abstracted thinking will determine how well you can follow along with how I explain things. In a nutshell, I can tell you the major steps to take along the design process and answer your questions while explaining the reasons why you need to do things a certain way in context. I know how to design things properly for functional parts, but I barely know sculpting. Sculpting could make this shape much faster, but the results would not be quite the same.

It is up to you. I don't mind either way.