Hi Marco, if you want to shell to the inside then one way to get it done is to use Edit/Separate to break your object into individual surfaces, then shell all of those to produce a bunch of individual solids, then boolean union those all together, then a bit of trimming off the bottom part and then a construct/Planar on the open edges and you should have a valid shape.

That's what I did to get the attached version - this one should now generate a proper STL file for you.

Let me know if you need more details on these steps or if you just need it of a different thickness let me know and I will make that for you.

- Michael