Why not get ahold of the traffic department where you live, and ask
them about buying a length of the pole they use to mount road signs?
Look around your neighborhood and you'll notice most of the traffic
signs are mounted on a piece of square tube that has holes every inch
on all four sides. It may be worth it to you to make some adjustments
in your project, materials, etc., to not have to drill all those holes.
Ronnie