USB Notebook cooler from Poundland £1
USB Charger from Amazon £3.49
Timer Switch from Amazon £3.25
Stick the Notebook cooler under the router, plug it in and set the timer.
I have set the timer to switch on for about 30min every hour from 9am-11pm.
Haven't measured temperatures, but the router does appear to feel cooler to the touch.
The Notebook cooler also has a nice LED which indicates when it's on.