I think the signs blanks your talking about are called coreplast and they come in many colors. I don't screen print, but weeding would not take that long to do. .... JB
Sintra's a bit malleable and more for indoor signage, since he said it was hard, I ruled it out. Acrylic is also known in the industry as plexiglass...Sorry, not sure what you mean by acrylic ,we use a material called Cintra that comes in many colours and is (I believe) a pvc based material. The UVC from the sun just eats it..unless it has a uvc protective coating, ie. a clearcoat or laminate,I suggest going with a green coro-either screen it or vinyl it. Shouldn't be a complicated job.
or use Diabond, laminate with vinyl then apply vinyl cut or screened image.