Anyone skilled in 3D modelling?

[Salty]

I'm trying to print this game map out on a 3D printer, but it won't print correctly because the insides are hollow and the terrain is so detailed.
So, using Autodesk Meshmixer, I solidified the object, but because the .stl wasn't modelled by myself and I got it off the internet, there are some issues with it.
 
For example:

The issue is is that the edges of the map aren't making contact with the ground itself, so when I go to solidify the object to make it non hollow, it deletes the bottom half of the map where the lip is because it doesn't know what to put there. How do I made the hollow edges on this object touch the ground so I can solidify it?

 

I'm using Autodesk Meshmixer right now, can't really use anything other than that.

Thanks.

[r1Ns3]

Sorry, not very skilled in this area but isn't there a way of filling in the hollow spaces under the mountains?

[Salty]

Sorry, not very skilled in this area but isn't there a way of filling in the hollow spaces under the mountains?

that's exactly what i want to do, but the 3d model as it is is terribly designed

[Nineshadow]

Something like this ?

 

I use Blender so...

http://www.mediafire.com/download/dam1sam8yfdd0go/untitled.blend

[infamous_last]

Haven't done 3d modelling in a few years, but a basic premise of doing it would be to draw an overlapping cube (slightly smaller than the surface), subtract the surface from the cube the delete the top half?

 

Only requirement is that the cutting surface has no gaps i.e. no holes

[Salty]

Something like this ?

 

 

I use Blender so...

http://www.mediafire.com/download/dam1sam8yfdd0go/untitled.blend

yeah exactly like that