Lost Captive Dust Nut Bushing

Was contentedly watching my machine when a screw flew past my face ricocheted off my Z4s windscreen and disappearing into the ether or the garage floor depending on gravity. The screw had worked itself loose from the dust-boot throat. A new screw I have but alas the brass bushing into which the flying screwed was threaded has also disappeared - fortunately without cracking the boot plastic.

Does anyone have the specs of the bushing or any idea where I can order a replacement?