Check out Coolbot. Whether you buy their controller or not, there is a lot of good information on that site about building a walk-in cooler.
From what I remember, the basics are:
4" of Foam insulation. Eventually fiberglass will get wet, even with a vapor barrier.
Buy an exterior door because well insulated doors are very hard to make.
No cracks. Not even tiny ones. You can fill in gaps with spray foam.