As I've found the 18mm is a bit prone to flare, I thought I would look for an alternative. What I found was a hood for a Mamiya 45mm lens for the 645 system. It is a 70mm push fit hood and fits perfectly straight onto the 18mm. I paid about £20 for mine including postage. This seems a typical price. The only problem is is that it's a bit deep so I took a knife to it and cut it back (rather badly as it happens).
The hood has a square section on the front. Not a lot of use with the 18mm as the front of the lens rotates so it's impossible to keep the hood square. Some experimentation suggested the whole square section needed removing anyway to stop vignetting on the 18mm so that's what I did. You end up with a round hood so it doesn't matter that it rotates.
Here's a few pics.
Some testing afterwards showed there was a tiny amount of vignetting in the corners so I trimmed another, maybe, 3mm off it which cleaned up the cut edge and got rid of the last bit of vignetting.
Being such a wide angle lens, a hood is never going to be very effective but it's better than nothing. I'll see if I can get some example shots with and without the hood to show the difference.
Just one thing of note, there is a Mamiya hood available that is said to fit the 645 45mm lens and also the 65mm RB67 lens. This is not the correct hood. The original version of the 45mm had an 80mm diameter front and that hood is for the original version lens. Check what is written on the side of the hood. It should be as shown in my picture above. If it mentions RB67 lenses, it's the wrong one.
(Edit: There is now a second part to this post with some examples).