Okay.. I see the problem.. there are two definitions of "Hollow".
Imagine a solid cube - it is made of six faces. If you remove one of those faces, the item is then "hollow", but the remaining walls have only a single (exterior) surface, and they have no "depth" or "structure" to them. What you are left with is a one-sided "skin" of a cube.. the walls have zero thickness because they are one-sided.
Now imagine a coffee cup. It is also hollow, but the walls have thickness, it has both internal and an external surfaces.
Your model is in the first category. It is a skin without any thickness to it, it has only exterior surfaces, and a big hole in the bottom. To print an item on a 3d printer, you must have some thickness to the walls. The amount of thickness for the walls depends upon which material you are trying to print in.
So, the question becomes: which material do you want to print this in, and how large do you want to make it?
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There are a couple of methods for fixing the model. Attached is the result from one method. I closed the bottom hole, created a copy, shrunk the copy by a bit, and then subtracted the copy from the original. That leaves a model that is hollow (per the second definition), with walls that have some thickness to them. The trouble is that this will likely be rejected because it has some areas that are too thin.
A more precise method of hollowing the item can be used, but I'd have to start with the item at near the size you want the final to be.
If I may suggest, play with the settings on my
Volume Estimator
and pick a size and material that produces a cost you're happy with.
More to follow in the next posting..