I get that unclamping the rails isn't an option, but why isn't it an option? I unclamp a saddle to switch between bikes right now, it takes about 3 minutes total from start to getting it angled perfect and tightened on the new post.
Its cuz I don't want to spend $70 right now for another saddle. Seems like a lot cleaner and more secure option vs shimming with home made or illfitting shims.
Hopefully someone on the wide web makes a shim that would fit properly.