Thanks Matt, jags,
I used a bottle of cylindrical shape and flat walls (non corrugated), 0.38 mm wall thickness (thicker that the original SKS flap 0.28 mm), this type of bottle renders plenty of flat usable material. It you cut the flap in the length of the bottle you will get a very convenient pre-curved flap that helps matching the curvature of the mudguard.
The disposable water bottles that I used are "Solán de Cabras" 1.5 liters, not sure about their world-wide availability (you can search for pictures in the web to get an idea about the shape of these bottles). These are bottles that many people reuse at home because they are well made and nice looking, with thicker walls than many other disposable bottles. They come in blue and pink colour, sorry, no black.
When you cut the slot on the flap I would recommend you start it using a round punch of the same diameter that the width of the slot to avoid creating square corners that could lead the material to tear up, though it is unlikely.
I am sure you can use many other bottles or containers that will do the job just fine... but "Fizzy Bubblech" bottles
Regards