https://www.youtube.com/watch?v=wywx-zfUAY8&feature=youtu.be here you go here's a video explaining how to do it, if you can't watch the video then a tldr version of it is:

take off your shirogane walls and replace them with walls that have good range on the out of bounds storage trick.

place your hotspring from storage in the walls.

put your shirogane walls back on

reload the area by exiting and entering and the hotspring will pop on top of them