Stop echo inside case

So with the cover off the system is much quieter than with the cover on. This is because the noise from my drives echoes through the metal box that is the HL15.

I tried adding some Dynamat type material on the inside of the cover above the drives and above the Motherboard/PSU part of the case but no difference in echo.

Any good materials I should try? Needs to be a few milimeters thin otherwise it won’t close.