Many museums attach plywood to handrails/ladders to keep people from climbing on the cars. Kill two birds with one stone.

Plywood can last for a very long time if it's dried and painted properly. Back in my naive youth, I replaced a bunch of broken out passenger car windows with 1/4" luaun plywood. (I fit them in like glass using quarter-round.) These lasted from 1987-2010. They were 2 coats of KILZ primer and 3 coats of quality latex.

Cheap and easy is laser print (not color - it fades) signs, cut the corners and laminate. The cut corners keep the paper from getting wet when you tack/staple it.

Or route the sign in plywood on a CNC router - paint the entire sign one color, and roll a contrasting color on the "high" parts with an ultra-low-nap roller (i.e. a foam hotdog roller).

If you are considering plywood, look up medium density overlay (MDO). It is used for outdoor signs.

MDO comes in one-sided & two-sided; the two sided is very weather-proof, err, resistant. The MDO surface is ready for paint, and if you use One-Shot Lettering enamel, it will last a long time.