Howto

See FreeCAD Lettering Tutorial for how to create extruded letters
Put a pad under text in FreeCAD, lower than text height
Add a handle
Export as STL
Note that image will be mirrored left to right if stamped as it. Thus - in Cura, Mirror the image. Use Mirror X if appropriate so your text reverses frm let to right
Print with rubber. Rubber allows ink to spread more evenly on paper. (Rubber or other ( Flexible 3D Printing Filaments ) ?)
Rubber stamp will print correctly with mirrored image.

Practical