It depends greatly on the environment (dry or damp), your location, ambient temperature, and access to areas to be insulated. Please give more information and a better answer can be provided.
You could investigate walk-in cooler panel construction for a start. The walk-in cooler panels have been thought out and it may be simpler to buy an insulated enclosure instead of inventing it if you are only building one unit.
Edwin22:
I agree with dvd, there are companies that design and manufacture both cold storage and freezer facilities, of the size he shows in his link, and entire large freezer warehouses. And, they are usually worth looking into, rather than trying to reinvent that whole wheel yourself. There are moisture and moisture vapor transmission issues, and foundation and structural element insulation issues which you have to contend with when you start trying to put significant cold storage space into an existing structure. And, in the size that you are suggesting a self contained modular unit is usually pretty practical.
