Here's something that may give you ideas, this is how I made my display...(check the last pictures)
how I built my streetlight display (the last 11 pictures (not in order) By the way for the curved arm (without the mounting part) look in the electrical aisle in hardware store, you will find EMT (Electrical Metallic Tubing (not the plumbing and piping area, usually in electrical...) you will see long metal poles look around that area you may find 90 degree ones on shelves....I used a 1 inch and a half....but now I recommend 1 inch and 1/4 for the arm only.....because the fixtures some can't go bigger than 1 inch 1/4 I once broke one of my NEMA's installing it in a 1 and 1/2 pipe......but some streetlights only accept 2 inch (usually the much older ones...such as OV-12 and OV-14B)
By the way to connect 2 poles like mine, instead of using connectors with screws...use those that uses compression pressure...i forgot what they are called, but they are connectors with 2 nut-like rings on them that tightens onto the pole, they will look better.....I have learned a lot along the way...