I need a cheap solution for a 3' x 8' sign blank.

Job info: Sign sits inside a wooden frame, and customer plans to replace sign in 3 years.

Any suggestions will be greatly appreciated. Thanks.

PolyMetal is good solution for that. If customer isn't too picky, Coroplast will last too and it is much cheaper than PolyMetal. 4' X 8' sheet of polymetal runs around $70, Coroplast..$10

I would almost go with COroplast, but the Polymetal will endure the elements as well asnd will be a stronger sign, which for 3 years is your better bet. PLus you can bend & shape it with normal woodworking tools.