It's an idea we've all had, and which many of us have implemented ad-hoc. In fact, my Airport Extreme is still mounted on pebbles stolen from a desktop zen garden, a measure rendered necessary because I piled books on top of it and it got very hot.
The real genius is, of course, getting down to the job of designing, manufacturing and marketing something useful to people who do not have desktop zen gardens to hand. Hence, the inexpensive Laptop Lifts, which do exactly what you imagine they do.
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Product Page [Laptop Lifts]