Give mesh a positive volume to create .stl from map

[mweinberg6J4YM]

I'm new to Fusion 360 and trying to understand how to think about a workflow.  My goal is to produce a 3D printable file (.stl) that includes topographical and building information from a square plot.  Here are the steps I am currently taking (I have no confidence that this is even the right approach):

1. Download a custom .dxf file from cadmapper.com (attached not attached because trying to attach it threw an error about the content not matching the file type - but it did open in AutoCAD without error. Not sure if that is relevant)

2. Open the .dxf file in AutoCAD and then export it as a .dwg file (attached)

3. Open the .dwg file in Fusion 3

 

At this point the topographic layer (Mesh 243) has an error that the mesh is not closed and does not have a positive volume.  I have tried to repair it without success. 

I think what I want to do now is either a) extrude the topographic layer down to create a positive volume, or b) use the topographic layer to cut the top off a cube that I create. Both of these could have the effect of creating a cube that is flat on the bottom and the sides, with a top that is defined by the topographic mesh (with the buildings attached on the top). 

Ideally, would then be able to cut additional holes in the resulting object.

Is this possible? Am I even thinking about this issue correctly?  I am very open to alternative approaches, or alternative ways to generate the file I am manipulating in Fusion in the first place.

Thank you for any suggestions!

[ababb]

not sure i have the same issue of posative volume on a mesh file i imported