Hi Lee, thanks for posting the file. So in your model one side of the hole area does not appear to be entirely planar. So I think that's why an extrusion from one side to the other is not joining in well.

If you use something like a Loft between the 2 sides that should join in ok in this case.

It's also possible to fill in holes like this by removing trimming boundaries, there is some information on doing an "untrim" here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

And there is a tutorial on this kind of process here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael

Thanks Death. Right on. The test now would be if you could boolean subtract from it now. No matter how I close it, when I try the boolean difference the result is an open mesh.

Thank you Michael - Makes sense. I'm still getting the hang of this, so my models are a bit sloppy.

I will visit the two links you provided. I appreciate the help.

Ok, I think your problem is that you do NOT have a solid when you do the subtract.
I used the surface and then blended the bottom edges (2 opposite edges, 0 Bulge),

then I did the same with the front and back openings.

Now Union the the thing and you get a solid from the which you can Boolean whatever you want...