Solution

After pondering the best (cheapest) way to remedy the problem of the airfilter cover an internet friend offered me his spare one for a song, the complete unit including backing plate came to less than the new posts I needed so a deal was struck.
I took youngest son for a ride out to collect it, a lunch out and a new pair of PC gaming headphones later I'm wondering just how cheap the solution was! Still we had a nice ride out together, and found several future day out destinations.