Based on http://www.thingiverse.com/thing:685585 but for Raspberry Pi B+. Because power usb connector located on a side, you will need to hack usb connector to fit between RPi and Fan.

For power USB connector - I took laying around micro-USB cable with a long micro-USB side. Using pliers - folded tip 90 degrees. This broke plastic base and, most importantly - pulled pins away from the connector end. So I used angle-cutter to grind off extra :) Sounds super hackish, but it works :)

As for getting power on RAMBO board - there is empty 5V source in the right bottom corner - see picture. Make sure to use correct polarity, as I believe incorrect one might damage RPi.