HELP--won't print apparently hollow object

Rhino Mac Tutorials
1

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?

Screen Shot 2020-09-22 at 5.22.38 PM
Screen Shot 2020-09-22 at 5.22.38 PM952×768 137 KB

Here it is exported as an stl

Screen Shot 2020-09-22 at 5.25.08 PM
Screen Shot 2020-09-22 at 5.25.08 PM1066×1166 105 KB

2

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