Looking to connect a piece of plywood between two boards to hang a banner
I am okay with the 5 gallons of concrete, that will weigh close to 100 lbs per bucket. I would use 1.5 - 2 inch pvc pipe, not 2x4's. That will give you better flex. For the banner, it is up so high that the wind loading and leverage will be a problem if you tie all four corners. So just connect the two poles with a rope at the top and hang the sign from the rope. This way in a wind, the bottom will just blow free. If you must tie the banner at 4 corners, you need a flatter container of concrete. An old tire, filled with concrete, will work for this purpose. Put a piece of plywood across the tire so it will hold the concrete. That's what we used for tether ball.
A five gallon bucket can hold .45 cubic feet of concrete when mixed with water. In other words, one 60 pound bag of pre-mix or an equivalent Portland Cement/rock and sand mix. At .45 cubic feet the bucket may overflow and/or be too deep to properly mix, but I do it all the time for difficult access pours of small quantities.
Hard to say. Just one 12' long 2x4 in a 5 gal bucket of cement should be rock steady, but hang a banner big enough to catch the wind between the tops of two like that and the whole thing could tip over fairly easily depending on the wind and the size of the banner.
