Since you can't get the loft thing to work the way you need (and I'm kind of a newbie with surfaces), you could detete the top and sides and just keep the bottom face. Then convert the bottom face to a polygon. Then extrude the polygon up to the desired thickness.

Better yet, just copy the bottom face and then do all I said with the copy. That way you can continue to try and figure out how to make the surface method work.