Hello Is a material called "copopren" http://www.leroymerlin.es/fp/15999312/panel-de-aislamiento-acustico-copopren-ca80-2000x1000x40mm-ign Here in Spain we use it to insulate walls. Its cheap and is very effective. Check for similar materials on a construction market near you...They must have something similar to this... Or you can go pro and use some especific insulation material matching the mid and high frequencies that your routers do. All insulations have a frequency range you must search something to reduce the 1000hz to 20000 hz range if its possible...This routers dont make to much low frequency noise...
Hello, I think you can use something like this attached to the router box. Try to avoid the direct to exterior big holes...