There are two shades made for the Contax G lens (in 46mm screw on) the CC-1 and CC-2 (from memory), a push on cap like the cv scheme was available for these, but a generic 55mm plastic clip on/snap in will work as well or better.
The CC-1 is ok on the 28mm Contac G Biogon (with a filter) suggest you need a mate with a G to try out his hoods on your camera, or a photo shop with 2nd hand stock.
The CC-1 I have is real pretty, I use it on my Contax II/Kiev, with a step ring, a step ring is your other option.
For the last 13 months I've only used a Kiev (or Contax), apart from folders, Fed's, Zorki's, M2, etc.,... and a digital to record dismantle sequences...