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i have an idea,use "Sonnet Rack-Pro-MM Computer Mounting Module for Mac Pro" to make a case for 6,1 macpro then DIY install fan and filter for it,will post the project after done.;)
 
Another approach is getting a small air tank for refilling at any gas station, or a little home compressor...,
and clean the Mac every 2 month or so.

(There are refillable air spray cans available, but they can contain only about 6 litres of air. So very little.
I don't know how many seconds they last.
They are usually to spray liquids like oil etc., and they use air to drive out the liquids.
Could be refillable with a foot air pump.)
 
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