After using a plastic bag as temporary raincover I looked at some commercial products. They seemed to have a simple design, not that hard to copy. I bought a sheet of tablevinyl (the kind that my mother used to protect her table linnen). Its fully transparant, durable, supple and doesn't crackle when the raindrops hit it. The photos explain it all. The sheet shown is 75 x 50 cm. The white spots are pieces velcro.
I use this cover for handheld shots as well. There is room under the cover to operate the camera with both hands.

http://72.167.78.60/DSC_0424 raincover.JPG
http://72.167.78.60/DSC_0423 raincover.JPG
http://72.167.78.60/DSC_0422 raincover.JPG
http://72.167.78.60/DSC_0420 raincover.JPG

Sorry, these are working links

http://72.167.78.60/DSC_0424%20raincover.JPG
http://72.167.78.60/DSC_0423%20raincover.JPG
http://72.167.78.60/DSC_0422%20raincover.JPG
http://72.167.78.60/DSC_0420%20raincover.JPG

That's a great idea Klass but you need to keep working on it. That' not enough protection in front from blowing/driving rain.

If Nemo swims by, you have to fold the LCD screen away and tighten the cover. If that doesn't help, you need an underwater housing. We experienced some real bad weather the last weeks and the cover holds up very well.

For those who want to cut a cover themselves I made a PDF containing a pattern and work instructions.
http://72.167.78.60/raincoverEX1.pdf