I assume you are breaking your system down, then reassembling each night, otherwise you wouldn't be having this issue. The expensive solution is a rotator, but that is overkill in this case. The sky doesn't change enough from night to night to make a difference if your properly aligned from night to night (I routinely shoot 30+ hours of subs that can take weeks to complete depending on clouds and moon). What you need to do is make sure you are disciplined in how you set up each night so that the image train is as close as possible to what you used the previous nights. Even just adding bits of duct tape to mark how your scope and camera are aligned will make the task so much easier. Also, try to set up your mount in roughly the same orientation. Again, before moving to a permanent observatory, I would leave marks on my set up location to make sure I could get to approximately the same position. Then your areas that don't overlap will be minimized and easily cropped. What doesn't work well is having subs at all angles. Then your stacks will show a mess and only the small center where everything overlaps is workable.
Hope that helps,
Jim
