Here's how I got it to work (I didn't figure any of this out, google told me):
- In openscad, export as .STL
- Open the .STL file in Inventor Fusion 2013. It should look correct.
- In Fusion, save as a .STL file
- Install the Inventor addin Mesh Enabler
- Open the .STL file (the new one from Fusion) in Inventor. It should look correct. Instead of drag and drop, I open from Inventor so I can set my units under 'options'.
- Right click on the part and select 'Convert to Base Feature'. Select output 'Solid/Surface' (cube) and check 'Delete Original'. Wait a bit.
- Now you can save as a .IPT file and you're done!