FWIW, I really like the Autodesk 123D program. I don't see where it exports STEP, but SAT works fine. I was able to quickly cobble together a basic widget in short order:


Create the basic shape in MoI


Export as SAT to 123D, fillet there using dynamic filleting, and bring back as imported SAT. Next add some kitbash:


Exports as OBJ to KeyShot and Voilà