We used used cooler panels for our first cooler and it is a great, inexpensive and quick way to construct your cooler.
Check craigslist or used restaurant supply stores for listings.
We had one client who wrote to us about cleaning up rusted panels. He used a big grinder and a wire wheel to clean loose rust off the bottom of the floor pan and primed it with POR 15, a rust stopping primer used in the auto industry.
Here is the before:
And after pics:
Some come with working cam locks, others (like ours) do not. We just used metal straps and used them to cinch the cooler into shape and screwed them in place. No thermal bridging to worry about here. Then we just used spray foam to fill in any gaps and cracks.
Check panels carefully for signs of water marks as that could compromise their insulating value.