Can't be bothered to fing the exact reg, but it is about ensuring there are no sharp object / part of laod that might injure others - do to with loads protuding beyond the vehicle I think. Need to be "highly visible".
Technically an oange sainsburys carrier bag would suffice.
I keep the prop covered both on tow and in storage - saves th prop getting dinged. Never had it come off. IMHO you're less likely to pulled by a bored policeman if you've a bag on - why give them an opportunity to examine your rig in (often ignorant or not fully informed) detail.
Like Whisper, I cut a hole in the bottom corner - lets any rain water out.
Member of the Macmillan Round the Isle of Wight Club