You could just try to adapt a reflector bracket. Bike shops often just take the front and rear reflectors off and chuck them in the bin. They should be able to throw a few your way.
Does anyone know where I can get these puny bits of plastic?
I've got a couple of boxes full of bits for bike lights, as I never chuck stuff away, and doing audax stuff gives me a strange compulsion to collect lights.
You're more than welcome to rummage through said boxes and take whatever's useful.