As it happens I am building a lighting board at the moment.
As other posters have said the ones generally available are, a) crap, and b) too short. I need a 2m one with a 8m cable.
I have sourced the lights from a lorry breakers. The ones on lorries are of much superior quality. I got a secondhand set for £15. Bought triangles, cable and plug from EBay for £12.
The big problem is the plastic board, I have had considerable trouble finding a suitable board. I have just found one from Greenhams, it goes under the name "Big Brother" barrier. Waiting for it to arrive.
I intend to make a custom mount so I can attach it to the A frame on the RIB. Probably a couple of bungees.
I have also sourced a number plate holder from a agricultural trailer supplier. This allows the number plate to be changed easily.
I haven't saved any money, but I will have a trailer board to my exact requirements.