Filling holes on the bottom of scanned design? I'm very, very new to Blender, and for an assignment, I've used the Trnio app to create a 3D image of a stuffed animal that I then imported into Blender. This is the result (as you can see, Trnio also scanned in the floor that the animal was sitting on):



The scanned bit of the floor aside, my problem is that the bottom of the animal wasn't scanned, as it was on the floor, and so I get this opening below:



My task is to both close the bottom of my design while also cleaning up a bit of the extra floor. But I can't seem to close my gap. I've tried selecting vertices, but this took a very long time and turned out to be quite a mess. I'm not really sure what the solution is, and would greatly appreciate some help.
Sometimes I've found that doing it piecemeal, fixes the issue, i.e. 


selecting the open area's vertices using ALT+LClick on an edge of the open loop
deselecting half of it with using Box select
press F to seal it with a new face, 
then selecting the remaining open half and hitting F to seal it