There are several App’s that can manipulate .STL files (if you’re not able-to / familiar-with CAD/other App’s for modeling).
I’ve designed many Molds for injection molding and Homemade molds for Polyurethane, RTV, Silicone and, Hot-Glue. It's fun.
Modeling your gizmo is preferred, of course.
However, if you have a .STL of the gizmo you want to mold, then, you can do this to make a mold:
1) You need:
1 - the gizmo .STL
2 - a Base .STL for the Mold base
You can use most any App to create the base and save as .STL. Most likely, simple are Rectangle/Box...
2) With those Two .STL parts (base and gizmo), find an App that can do Boolean Subtraction’s (Differences).
Adjust the Height & Placement of the parts as needed then,
Subtract (Difference) the Gizmo from the Base to get a pocket of the Gizmo’s shape.
Naturally, if you want a ‘Split’ mold, do it for the Upper and Lower mold sections.
You can make a hole in the top section and pour in the mold material, if not simply using the bottom section and pouring goop into it. Clamp the two sections together. Make an Air Vent hole too for displaced air.
I did this example using FreeCAD for the above approach (modeled it and exported .STL of Base and theThing. Normally, I would simple make the full mold in CAD).
I then loaded the .STL’s and Differenced them to get the mold.
Of course, there are details you’ll consider, such as scaling to compensate for material shrinkage and part-ejection…
3D Print Parts