Just as the other guy mentioned the wind force is going to be two great you'll have to make a sturdy frame with a backing that is going to be bolted to those columns. you build a solid frame because they want these the last long time then it'll be easy to put the banner on there in the frame takes all the abuse not the banner. But wait your customer only has a $3,000 budget correct?
This is a big concern and you need to talk to the customer about their goal and the feasibility of what they want to do. many times the customer has an idea but you need to present a solution that will deliver the result but is not exactly what the customer asked.