Here is a rough sketch of a cooling coil installed in a fermentation vat
It is meant as a guide and schematic model.
Use your own tchnique to build a coil that will cover a large portion of the vat.
This will give good results as long as the mash is stirred occasionally.
Note that the coil should not pass through the bottom of the vat.
Bend the end of the coil so both ends of the coil are at the top of the vat.
Image source: Mother Earth