Many folks consider building a cooler themselves to “save” money but there are some disadvantages of building your own. The cost of a self-built versus a good used cooler is almost the same cost typically if you account for your labor. The advantages of a prefabricated cooler are quick installation, usually 1-2 hours, and it can be quickly taken down and moved or sold if needed. A built-in-place cooler typically will not be able to be disassembled and moved easily.
Only foam board or foam-in-place in should be used for the walls and ceilings. The inside environment in the cooler should be very high humidity if you are to keep your produce from dehydrating. Even the best construction will have holes into the wall and ceiling cavity and moisture is the enemy of fiberglass or loose fill insulation. A year after construction the insulation value of the cooler will likely be half the insulating value if you use fiberglass or loose fill, increasing energy costs and slowly deteriorating the structure. Foam board insulation will retain its insulation value for years.
The following tool can be used to assess the cost of a self-built cooler.
Description and link to construction for a self-built walk-in cooler by the University of Kentucky
Use Foam insulation in place of the fiberglass battens the plan calls for.
Link to detailed plans: http://www.bae.uky.edu/ext/Specialty_Crops/