How can I make a closed flat base for an object? I've made a 3d model of an ancient sculpture with its nearest surrounding. Since I want to publish it on Sketchfab, I need to make a base for it, make it nice and clean, plus reduce it's size to make it more comfortable for further review.

This is how it looked in the begining:


And this after extruding edges:


Better but there is still long way to go. This is how I do have some questions. Hope You can help.

I was able to extrude edges of the model along z axis to make a base. But - even I was strictly following the tutorials - I was not able to create single face that would close the model from below. Are there any requirements regarding side faces that need to be fulfilled to get this done? Since the mesh was created in Photoscan, not blender - it does not have straight edges, but it resembles saw or something (unfortunately). Is this a problem? 

Thanks in advance!
In Edit Mode, select all the bottom vertices and then press SZ followed by 0 then Enter.

This scales them all to zero on the Z access which will flatten the bottom edge.

Now press F to fill a face for the base.