Those objectives sound contradictory to me but if I were to go about doing it I would have a timer of a month, afterward the claimed region (chunks as I use Lands) would regenerate the chunks and be deleted, while the build gets stored in a separate system waiting for the person to come back. If the person comes back later the build will be replaced back onto the same spot if nobody else has claimed it yet, but if there are claims overlapping where the build last was then it will give the option to place the build elsewhere using a tool like a feather or whatever you assign which aligns with the chunk dimensions