Yes, like the well-know Kensington System Saver, thankyou so much, I'd never heard of them

I meant homebrewing one that doesn't require the mains power supply and consequent excessive size. Flush with the top of the Mac and no massive protrusion out the back either. A simple, small box with a 12V fan - perhaps underpowered at 5V off the ADB port, or off a disk drive power splitter from inside.