Well, you could create blocks or aspects that would help on your defense. But if it's something you want to do an a regular basis, I think it might be easier to take a toughness power and link it to human form. Activating the power would be narrated as a spell, but wouldn't necessarily have to be one.
So it would look something like this:
Human Form [+1]
affecting:
Supernatural Toughness [-4]
The Catch (only protects against things a regular armor would protect against) [+2]
------------------
Total: [-1]
That way you have something that's covered by the rules, and you don't have to manufacture something with evocation.
How it looks is another story, and would probably depend on the wizards style. A block of rock, vines twining around him, his skin turning to steel, his clothes turning to platemail, and so on could all be done. Craftymanship only plays a minor part, I would think, as the magic translates the wizards thoughts into an item directly.