I don't fabricate them myself, I use seeed studio in china for that, mainly. I design them and assemble parts onto them myself.
Funny - I came home in heavy rain a few days ago on the highway, both lanes were blocked and I was in the left lane. Had an asshole behind me who was both tailgating and flashing their high beams, so I started a mental design for pretty much exactly what you're describing.
I was thinking of a screen with a red LED dot matrix that would display 5-6 letters max, placed against the back window of my car. Up front I'd have a few buttons on a controller that would make the screen flash out text, one word after the next.
BACK
THE
FUCK
OFF
or
TURN
OFF
YOUR
HIGH
BEAMS
etc...
To get stared on the sign, I'd probably buy a cheap scrolling LED sign off eBay, and put a new controller in it that lets it run off automotive 12V and has the button panel connection on it.
Building such a thing into a tailgate might be tough/expensive (requires everything to be sealed against water/moisture, running a cable into the tailgate, etc)