OK I have a couple of points to make:
Firstly most PSU are full of slits and air gaps in order to allow air to circulate... this will be a task and a half to seal plus the fact it opens up and you'll have to deal with the joint between the two halves.
Secondly if you did dispurse the heat around the PSU you have to remember that parts that do not normally heat up will start to rise to a higher temperature than they were desgned to handle. In electrical terms becoming more resistive. This may lead to fuctuations in current.
I doubt you'll get better cooling and quieter than just modding your PSU with a single 5v 120mm fan mouted onto the top.
~ Boli
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