Light foams are only last touch.
First thing to do is lower resonant frequency of panels and increase their acoustic opacity and vibration resistance and that requires mass, like bitumen mat used in car acoustics. Light as air foams should attched only on top of that, and thickness should be at least about 1cm.
Also when there are direct noise escape paths like in most cases you can't do much anything to "airborne" sound because most of it escapes directly out any way unless you block those holes.
That's why proper case design is important for silencing.
BeQuiet Noise Absorber is probably cheapest meant for PCs combination mat which has heavy mass damping layer and acoustic foam both in reasonable thicknesses.
Again if you want DIY approach just get some dense bitumen felt/mat/whatever it is called in general stores and then some acoustic foam separately.