HELP--won't print apparently hollow object

So I have created an object which is a closed polysurface with no naked edges. However it is not able to print because it is hollow and aside from one small area there is no where for the joints to be added in order to print. The joints just disappear into the body of my arch. Is there a way to fill my object?

Here it is exported as an stl

You will have to post a Rhino file with your NURBS geometry, otherwise it is impossible to help…