Quick question, anyone got a good scheme for key stretching?

I have this scheme for managing private keys that involves storing them as
encrypted PKCS#8 blobs in the cloud.

AES128 seems a little on the weak side for this but there are (rare)
circumstances where a user is going to need to type in the key for recovery
purposes so I don't want more than 128 bits of key to type in (I am betting
that 128 bits is going to be sufficient to the end of Moore's law).

So the answer is to use AES 256 and stretch the key, but how? I could just
repeat the key:

K = k + k

Related key attacks make me a little nervous though. Maybe:

K = (k + 01234567) XOR SHA512 (k)

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