you can try using blender, which is open source & free
I did mine with blender as well, all the info/instructions I found on this forum
if I remember correctly:
export from google sketch as Google Earth 4 *.kmz file
change the file extension to *.zip
unzip and find the model file inside directory 'model' (a *.dae file)
install blender (might also need to install python to get the importer to work)
might also want to install meshlab and run to clean up the model / check for non-manifold edges
import dae file in blender
select object
-go into edit mode
del a few faces to make holes in out of the way spots so 'hollow' material can drain out
select all
mesh - scripts - solidify selection (close edges)
play around with the solidify value to get the right thickness
export file as collada dae file (or *.stl but I haven't been able to get that exported file to work)
I think there's also a tutorial on how to hollow out a model, in one of the recent blog entries:
https://www.shapeways.com/tutorials/creating-hollow-objects