There is no "Extra Defense" advantage that you could then modify with, say, Independent or some other modifier that would make it hands-free. However, you get one additional block or parry per hand, regardless of number of hands or number of attacks, so that's where the solution can be found. Try this:
Extra Arm 1 (Force Extension, +50%; Shield Mount, -80%) [7]
Force Extension is stolen from Stretching; see
Powers, p. 78. In this case, it gives you a visible limb of force that can't be injured.
Shield Mount is by analogy to Weapon Mount. If being restricted to mounting and using one weapon is -80%, the same should go for any other dedicated-purpose limb.
You can make it invisible with No Signature, if that's important to you. I don't see how it's worthwhile, but whatever. Since you can always specify special effects for your abilities, why not just say it has the appearance of a glowing shield?
As for the shield itself, I'd allow the arm itself to count as a shield to block -- it's an indestructible force! Use the standard Parry for an arm as your Block score, which is DX/2 + 3. If it needs to be higher, buy Enhanced Block (Mystic Shield) [5/level]. Enhanced Block requires a specialty, and I think this is fairly specialized!
To give, say, a wizard with DX 12 a first-rate Block of 16, you'll want Mystic Shield [7] and Enhanced Block 7 (Mystic Shield) [35]. I'd say that 42 points is a fair price for one sure-fire, stop-anything defense a turn. (Especially when you compare it to Blocking spells . . .)