Tulou cluster mostly scatter acorss small towns in Yongding and Nanjing county.
From Shenzhen , you can take a sleeping train /K636 to Yongding county 21:23--04:43. This train can aslo bring you to Longyan county which is the next stop after Yongding.
Then transfer a local bus to the towns where tulou are located.
If you don't wanna pass Xiamen, take this train come and return would be the best way.
But you should be ready for the crowded tourists because you would come during national holiday~ haha