This is a great question.  We have used and seen numerous shielding products and contraptions designed for this purpose.  The problem is that using such devices will bring very, very mixed results.  Depending on the type of material, construction, layers, and holes/gaps (such as those required for wiring) - the results will be very mixed and unpredictable.

This is one of the reasons we really like our Low Emission WiFi Router

Rather than attempting to shield, we have changed the source.  Rather than try to cover up or block, we have reduced those emissions to begin with.  We have cut down on the radiation by approximately 99.8%

Plus, you have the ability to adjust it.  You can increase or decrease the power of the router.  So don't try to cover it up - eliminate the source :)

Is there any special modem you suggest to go with this router?