If you are interested in creating items that are more artistic than industrial, then I suggest that you have a look at ArtCam, http://www.artcam.com/

This is a relatively inexpensive (about 130 Euro) software that can be used for designing things like jewellery, badges and such. Things that have a lot of internal lumps and bumps.

The software was developed for use with Subtractive Manufacture (that is, cutting away material from a block using a CNC milling machine). However,if you use it to design an object, you can save the design file using one of the CAD fie types, then import the file to your slicer program to create the Additive Manufacture G-code.

I have ArtCam installed on my workshop computer, which is a way away from where I'm siting now, so I can't check if ArtCam will export designs as *.stl files.

If you go to the ArtCam site, you can download a trial version to see how you like it.

Old Man Emu