I suspect I've posted this already, somewhere, but...
I use strips from 4 pint or 2 litre plastic milk bottles: -
Take off the label.
Cut around the bottle along the ridges above and below where the label was, to provide a loop of plastic.
Cut across the loop on the line under where the handle was and open out.
Use hot water, hot air gun (not too much heat) or maybe (haven't tried it) domestic iron protected by a thin sheet of cotton to soften the strip. Lay out flat on a suitable surface, preferably with big flat object (e.g. bit of wood) on top.
Cut off wrinkly/ridged edges with sharp knife & steel ruler.
Attach to mudguard by whatever means you fancy. I have used pop rivets, with or without reinforcing strips of plastic or thin aluminium alloy. I'm currently using the reflector which was originally mounted a bit higher up the mudguard. Gaffer tape should be OK, though I've only used it on front mudflaps.
Aerodynamically it has to be the best solution since at speed it streams out horizontally. However in that position it still stops the road dirt from flying up into the face of the following rider (so I am told by the few riders who have followed me).
In terms of appearences, it's not going to win any prizes, for the simple reason that it's permanently covered in mud or animal excrement. Anything that isn't probably didn't do its job successfully.
Photo taken today.
