My 500MBWE (single drive) without fan was running quite hot about 55c
I have sat an XBox twin fan on top which plugs into the USB and it now runs at a cool and steady 35c which I am much more happy with.
The unit fits nicley on top (although a little larger than the MyBook), and at just $10.80 delivered its a nice sollution.
Thought I would share this if it helps anyone.
These are my stats shortly after turning the fan on:-