The requirement of the water tank:

1. the size should be as small as possible 
2. able to fill up the insole
3. the cooling module should be removable for a more powerful cooler or other purposes
4. good contact with the cooler surface
5. no leakage

The design of the water tank is below after testing

Experiment setup

The water tank shape could be further adjusted and also the way to couple with the cooler to get better performance. Since I could achieve the aforementioned requirement, I decided to move forward to see what happened.