I would put nothing between the shelf itself and the plastic tray. As long as you have a bonding sealant along all of the flat surfaces that will be in contact with the shelf then all should be well. Sound deadening needs to be under the tray in the engine compartment, or a thin layer on top. In my van there is a very thick foam pad in the engine compartment. My van was one of the last made so I don’t know if this was common to all, or like some other features a later addition.
cheers Tony