Erm Perspex a hack saw, course/fine sand paper, hot hair dryer for shaping a little of needed, and some velcro to stick it on with.
Simple bike modification, just make a template of the required size and shape (add a little for the velcro)
Get some clear 2-4mm Perspex and cut it to shape with the hack saw, then rub down the sharp bits.
Use the hair dryer if you need a slight bend in it, and then stick on with sticky velcro, that ways it's still easy to clean you light.
Quick simple cheap and a WORTHWHILE modification.