There is also the braces that the seatbelt assemblies hold to as well, those have to have a little weight to them as well. I am going to get a spotweld drill and remove them, has to be good for a few more pounds. The seat bottom is almost weightless, but the seat backs have some weight, but a lot of it is in the seatbelt rolls, brackets, and other BS.
just make one out of lightweight plywood... another thing to drop weight is all the insulation that is glued to the floor in the back, if you don't mind the extra noise