I built it last winter. Here's an action shot. It has a single lower tube and two small upper tubes. If you build one, remember to keep it light. Every pound it weighs is one less you can lift. You run out of hydraulic power pretty quick on the end of an extension gin pole. As far as counterweight, if you presently have enough to keep from lifting the back wheels off the ground when you load the bucket beyond what your hydro can lift, you have enough for an extension.
The lower tube carries the load in compression so it must be stout. Mine is braced at mid point by the very small (1/2" square! it's so small and cute) braces. This way it can resist bending better as it is compressed. The two upper tubes are in tension, so can be smaller. Could be rod in fact, but I like tube for more welding area. A small amount of steel will support a great tensile load.
My loader is quite small, with only 6' lift. The gin pole got me another 6 feet for raising headers and beams, and it worked absolutely fantastic for setting poles in drilled holes in the ground.