Provided a) you do not carry many people and b) you have a two door this solution worked for me..
The back rest became the base in my moggy, the seat cover comes off the back very quickly, I then screwed a piece of wood on to the metal back that butted up against the lip on the box section that the original seat sits on. Using wood has the advantage that you do not have metal to metal contact and is therefore quiet... nothing to do with the fact that I can not weld!

This leaves a small area at each end of the seat, I just made some little storage "cubby-holes" out of glass fibre to fill the gap.
The base of the metro seat then got ripped apart to form the back rest. I cut a piece of mdf up that fitted in the car, drilled holes so that the original bolts could go through and hold it all in place. I then cut the foam of the base to the new shape, pulled the fabric over and used a staple gun to hold it all in place.
The problem is that the ride in the back is not comfortable but as the car had no suspension travel and I can count the times I had people in the back of the car on the fingers of one hand this was not a problem.