Don't bother with the hood.
I went on the cheap and bought a Contax G hood that screws in for $12 out of Hong Kong (be prepared to wait 3 weeks). No success.
Not happy with that, I found a proper Zeiss Biogon hood for $40 on eBay, they list for around $90.
I was not a happy camper with that either. Although the Biogon is VERY flare resistant the old rule of 'sun over your shoulder' still applies to wide angles over 35mm.
With the proper hood I still get a moderate veiling flare from high noon sunny day shots, but the flare is very similar to that of the human eye where detalils are washed out in the shadows. Usally a hood will lend that baseball cap look to things, but not with the wispy hood required for such an extreme wide angle.
I used the factory hood shooting 90 degrees from a sunrise in a desert landscape and still got curtain veils running vertically through the shot.
The Biogon handles flare better than any wide angle I've used, but it still has its limits. I don't find hoods to be of much help with wide angles because they have to be too small to accomodate the rangefinder. Save your money and practice proper techniques.