So, we have the Hingan Hot Spring and Alpine Open-air bath, they are neat and all, but is kinda small though good for a group of 4 people to relax in.

So, what about Hingan Outdoor Pool (Carpenter) and Alpine Outdoor Pool (Goldsmith)?
The size would be a little bigger than the wooden deck - where the pool itself can fit into the wooden deck if you overlay the items when placing them, and the edges of the pools would be just a tad bigger than the wooden deck.

The question remains would they add some depth to the pool so you could actually swim in it? hmm.