I'm finding with the Plus 4 sport not having a tool tray in the rear, the exhaust noise/boom seems to enter the cabin more and a little tiring after a while. So I've put a removable acoustic membrane (material I had over from flooring) over the top of rear parcel shelf and battery cover and under the front floor carpets to just take the edge off the boom. A little bit crude for now, but its seems to help, so I will in time find a more appropriate permanent solution..

22 Plus Four KIMI
12 Plus 4 Sport OZZY
08 Roadster FELIX
06 4/4 70th LOKI
77 4/4 SEAMUS
85 4/4 MOLLY