These are models of the individual lumbar vertebrae (L1-L5) and the sacrum, based on the 3D models available in the bodyparts3d database.
I cut the models in half to facilitate printing, just make sure the flat side is down in the prep, then print the two side and glue them together. Comes out quite nicely, the original models are very well done.
In the pictures, I have the L1 vertebrae and the complete L1-Sacrum. These models are full (life) size. Although the models don't look it in the thingiverse preview, they are closed.
The BodyParts3D uses a creative commons/share-alike license.
Credit to Cbonsig for pointing out this excellent database and putting up another nice version of the lumbar spine.