Polyfoam won't work as it not absorb the sound well. But still worth to try as it's free..
Try ask from car accessories shop for sound dampening mat. Pick most thinner (1/4 - 1/2 inch). Maybe can got more cheaper compare from computer shop.

To minimized fan humming sound, do not obstruct airflow with thick filter or smaller fan grill holes. Better if direct opening. Put rubber washer for each screw. That how I do...