Advice on how to make my model manifold for 3D printing in Cura

Hi all. I am student working on a project trying to 3D print a physical model of a design I created in Revit. The design is a topo-surface from revit with a few objects on it. So far, I have been able to export the revit model as a DXF CAD file and import it to Sketchup and clean it up a bit with a plugin. My issue is that in order to print I need the bottom of the model to be a solid flat surface to be accepted by Cura and print properly. I have included the Sketchup file so you can visualize my issue. Any suggerstons on how to go about making this printable? CleanedUp.skp (923.6 KB)