Ouch. Sorry to hear that. Hope that you have some success when you have it done again.
In the end, I abandoned the idea of retrofitting something when I purchased a very well-made durable air cushion from Relax the Back. (I didn't like the color mis-match, so I had a black pillowcase sewn around it to match the color of the seats.) It works quite well on long-distance trips.
Lesson learned: if I buy another BMW, I am just going to suck it up and pay the extra cost for whatever package has the lumbar supports.