When did standard metric thread wingnuts become hard to find? I bought one to secure the 240D air cleaner cover just a few months ago.

But like i said, omitting the nut entirely is probly the way to go. In fact it's tradition on 111 sedans to hobart the securing nut because that one nut alone is the only reason you have to reach behind the dash for pulling the speedo instrument cluster. Never heard of one shaking loose without the securing nut.