Printing a 3D model once it's "water tight" is great, but expensive: The material (and time) used to print a solid object is might greater than a hollow one with a connecting "spout" to the outside.
For this object - my original print was 22mm wide, but cost ~$10 via Shapeways.
I wanted to upscale the model to 28mm, but keep the cost from increasing drastically. With the small hollow area inside, the larger version only cost ~$11. Not a huge difference, but when printing multiple models, that space adds up.
Using only free software (Meshlab and the free version of Netfabb), you can create hollowed out models for printing:
It's also possible using Google Sketchup to create a simple object (a sphere, or custom drawn 3D object), then to 'flip' it's faces. This makes your inner surface. Drag / position this inside your 3D object (in the same workspace), and punch a hole in the polygons and draw a 'spout'; connecting the flipped inner shape to the surface of your main object.
Since Sketchup doesn't automatically turn spheres into polygonal objects, I had to draw the veracities for the 'spout' entrance directly on the sphere. Before, if I tried to delete the blocked area in Sketchup -- it would delete the whole inner sphere (because it was a solid object - not polygonal). This meant I needed to draw directly on the sphere with the line tool, then connect that to the outer shape.
Once that was finished, I exported the whole workspace (Sketchup .obj > to Meshlab .stl >) into Netfabb - and deleted the (now polygonal) area blocking the spout. It's a more labour intensive method than the automated meshlab version - but necessary if the automated process won't work for your object.