Which manifold insulator would suit with my Holden Commodore VX.II Ecotec L36 (Auto)? I would prefer to keep my engine cover.

If you're adamant about keeping the engine cover then I would recommend sticking with just the 12mm manifold insulator. While you could run both the 12mm plenum spacer and 12mm manifold insulator and maintain your engine cover, it and or the bonnet liner would require to be modified.