If you use the composite, get the tongue and groove to hide the attachment and if you put a metal backer as Rick shows you should figure out a way for moisture to wick off so as not to have further problems later on. Vertical shallow saw cuts on back side of board for water to drain down.
The composite siding has all this engineered into it to wick off moisture but then that has an angle and not flat as your pic shows.
It's a good looking sign the way it is put together. Good luck.