I've tried the 3d printing route, but unless you are running a farm, it will take a week to knock out a set of 10" letters, as each one nearly takes up the plate. Now with 4", if you can nest them it'll be about 8 hours. But with a good cnc, you could pocket out HDU in 30 minutes.
Also I never got as far as putting something outside to check on weatherability, but since it was printed polycarbonate, it should have held up decently.