OK, I've found a cheap, easy solution for mine. A guy at the shop had a spacefiller made for him - cost about $20, and mounted to a form fitting vinyl pillow (could have been leather - not sure), which filled in the space nicely. After a while, I remembered that I had a similar sized item collecting dust in the garage. A couple of years ago, I bought a seat cover/vibrator for my 2005 Mustang (which had horrible lower back support). It had a cushion for the lower back which strapped into the crease of the seat. Sure enough, it fit into the gap just about perfectly.
The first picture shows the gap. The second is the lower back portion of the seat cover, and the third shows it installed. It is a nice tight fit, with no possibility of falling out, although it does have a velcro strap on the back, if necessary. My wife tells me that lower back support is noticibly improved when it is in place. As a bonus, it also has a built-in battery operated vibration mechanism with an on/off switch.
Ed
That item you purchased for your Mustang a few years ago is a vibrating headrest. I saw them at Walmart tonight. It looks like the identical dogbone shaped pillow that you showed in the photo. The middle section is some type of pleather, but the outer portions are more clothlike. I would think that could be a problem if it gets wet. Walmart also has a vibrating lumbar support, made by the same company, that looks like it would fit the entire area below the passenger backrest perfectly. Only problem is that it is all cloth on the exterior. I would think that this item would be perfect if it was covered over in leather or pleather. You could then place several velcro strips on the backside of it and the front side of the drape, which would hold it is place perfectly, and then just rip it off for easy cleaning access. I am posting a link to a similar lumbar support from Walmart next, but it is NOT the one I saw.
http://www.walmart.com/catalog/product.do?product_id=10161135 The reason I post this is that it looks similar to this in shape, but I did not see the vibrating one on their website, nor the headrest on their website. The one I saw looked to be about perfect size and shape for that area. If I would have thought of it at the time, I would have taken a photo with my phone in the store.