You can use any software to edit the cap before printing. But Tinkercad is just enough for this purpose and fast to edit.

Use any vector software to export and svg file and upload it to Tinkercad, you can create a hollow shape with this svg file. 1mm is enough.

For 3D printing you will need to set layer height a 0.2mm maximum to get a good resolution on the thread part of the cap.

Disassemble the arm of the binder clip and put it inside the cap.