I’ve been playing with a few ideas for an 8mm gate, mostly influenced by the blackmagic cintel gate.
The top 12mm length is flat, and then falls off slightly.
The underside is cut to match the the size of the integrating sphere below to bring the gate as close as possible.
The guide bearings are offset from the aperture so the screw holes don’t punch down into the sphere (I made a mistake already)
Ideally the two bearings on the operators side are adjustable by pushing them in via a set screw to adjust for film width variations, but I haven’t got that far yet 
The groove cut is just the width of normal exposed image area.
Hopefully once I sort out a design that works I could get it machined and chrome plated.