The Movcam 104:85mm Step-Down Ring for Clamp-On MatteBoxes lets you mount 85mm lenses onto 104mm clamp-on matteboxes -- the kinds without a thread on their rear adapter.
The ring is split along a point of its diameter to flexibly widen and mount a mattebox. (A locking pin on the side of the mattebox securely tightens the ring in place.)