zoon in very closely to the model's vertical centerline. You will see that the surface edges are not planar with each other. There are tiny overlaps which need to be trimmed back...

how to fix it... Once you trim the surfaces back, you can create surface blends between two opposite faces (either the top and bottom pairs, or the left and right pairs). then create the next 2 surfaces using Sweep. Join all together, and the boolean should run fine... But i think your model could be developed much cleaner using a simple revolved surface for the top and bottom faces, then using a curve to boolean the shape from the top...

jonah