I'm not sure I understand. Is your saddle all the way back on the current seat post and you still need more. Or are you trying to avoid clamping your saddle more forward of the middle of the rails. If the former, you need more setback. If the latter, just push the saddle back. Contrary to popular belief there is no reason the clamp needs to be centered of the saddle rails...or even close to that. If you need more setback, there are seat posts with 135 mm. Here you go for a suggestion: Specialized Alias seat post with 35 mm setback. Without getting some monstrosity, that is about as long as you can get.