How do I extrude out a base on this model? I have this .obj file in which the model is made from pictures taken from the front (in a semicircle). This results is the model not being an enclosed item, but rather more like a folded sheet of paper:



I wish to print this model out in 3D but to do that I of course need it to stand on its own, which is why I want to "build" a base for it. I looked it up and the way to go seems to be extrusion, but whenever I tried to extrude every single detail proportionally which results in a big mess of half-enclosed spheres, stretched out and attached to a jagged paper.

Can anyone tell me what I'm doing wrong and point me in the right direction? If you need my .obj .mtl or texture .png file to look more closely, say so!
And, if there's someone with too much time to spare, would maybe that certain someone be interested in doing this for me? Any help is welcome!
If you want the base to go straight down then extrude on the z axis, then press 0 to make the base level E,Z,0, then grab on z (G,Z) to bring the base down to the height you want it.