The foam is not there to filter or bleed air into the vacuum chamber but acts merely as a spring to keep the one way valve closed just enough to leak a bit of air in, so you can use any foam you like as long as it's springy enough.
Car wash sponge is too soft, but pipe lagging might be just right as it's fairly stiff. Put a chunky bit in first and see what delay you get - then you can trim a bit off to fine tune the delay afterwards.