I don't think anyone implied the light board was 'unlawful'...?
They pointed out the need for two triangular red reflectors. I quoted the dot Gov link purely as a quickie......reminding folk that triangle reflectors are not lawful on a vehicle...only on a trailer.
Sticking a pair on the mudguards might suit....but do check out the angles of visibility required?
Which reminds me..I need to get a pair of white reflectors for my own weeny trailer....somehow, ex-military glowy sticks tie wrapped to the mudguards don't quite hack it? Especially when I've only got red, green, and infra red!!