For anyone in the future that reads this, here is a place you order a custom made adapter that converts 6 x 12 vent to fit a 6 x 14 port
http://www.ponoko.com/design-your-own/products/612-614-fit-kit-for-keen-home-smart-vent-13176
Make on a account on that website and add it to your designs and click
http://www.ponoko.com/products and select the design.
It cost me ~$5 after coupon and ~$7 in shipping cost for total of $13, ships from oakland, ca