Look into locally sourcing a used walk-in cooler. I think we got our last one for the cost of taking it apart and hauling it off.
Yes, you can build your own. But you can most likely buy a used set of panels and door for less, and it will perform better. Getting the walls 100% air-tight is critical, especially in a tropical environment. This isn't easy. Sprayed-in-place foam would be the best option, and for the cost of that, you can buy a new walk-in.
Timm Turrentine
Brewerywright,
Terminal Gravity Brewing,
Enterprise. Oregon.