Maybe you could just write the last bit of your book. During November, make sure you need at least 50K before you get to that part, then tack it on in December.

Don't know if that's a practical suggestion or not; depends upon your story. But thought I would mention it.

...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown