For anyone interested in improving the stock seat on a GSA (should be the same for a GS also) this could be one option. The stock seat on my GSA, while not horrible, wasn't all that comfortable either. For me the two problems with it are the downward slope toward the front and the slight crown side to side. But the stock seat looks good and matches the rear seat. Having someone like Rich's, Russell, or Mayer remake the seat is expensive and I'm left with a re-covered front that doesn't match the rear. Having them also recover the rear to match is more expensive yet.
Looking underneath I saw BMW's upholsterer's left a little extra material so I decided to try and modify the shape yet keep the original cover. I showed it to a local car/boat upholsterer and explained; "I want to sit at the rear and don't wish to go any lower because I like the knee room, and as it is the rear meets flush with the passenger seat however I'm constantly sliding forward". I asked him if he could build up the dish with dense foam and flatten the side to side crown? He said he thought he could and showed me some very dense foam he uses under convertible tops that would work. I told him to do it even though I was concerned the line between the sand and black might end up slightly curved. He had it done in a couple days and charged $140. After just a brief ride the improvement is dramatic. Plus it still has the factory look and overall shape, and matches the rear.