Here is where I got the aluminum for the upper shaft portion of my custom shaft. It's an Unger professional window cleaning pole and can be had for $20 at Home Depot or probably Lowes. The picture shows a blue color but all the Ungers I've seen are green. Etore is blue, and I wouldn't use the Etore as they are heavier so the aluminum is probably a thicker grade. The inner extension pole is the one to use. It's the same diameter of the stock upper GT shaft but much lighter in weight being a lighter gauge, yet it's stiff as heck. Has to be to withstand extended weights of 10 feet using a mop head and other window cleaning tools.
The beauty of buying this to use the aluminum for the upper shaft is not just weight savings (use a Whites tall man carbon rod for the lower shaft), and not just that it's found at any local home improvement store, but also because you get an extra snap pin clip with it as that's how the top plastic green tool holder is held in place on the shaft. Cut off a section of the inner shaft to the desired length for your Sovereign (much shorter than stock if using a Whites Tall Man Rod), and you also will still have a slightly shorter but still extending window cleaning pole for around the house, changing light bulbs, dusting, painting, and other things. Much more value then buying some blank aluminum stock through mail order somewhere by the time you pay for shipping.
Interestingly enough, Unger also is selling a carbon fiber version of this pole, but they want way too much for it. I know of real cheaper sources for carbon via mail order should I decide to replace the upper shaft with carbon for maximum weight savings.
This grade aluminum is meant for constant outdoor use in the elements for years. My original Unger pole had about 15 years of use in the window cleaning trade and hasn't oxidized or fell apart on me. They may sell shorter versions of this pole cheaper but I haven't compared them to see if they are the same diameter as this one.
You do not need a cam lock. Just drill a hole completely through the aluminum for the snap pin's two heads to pop out of and it's going to be plenty stiff enough. From my experience Camlocks strip quick and don't do squat for rigidity of the shaft.
http://www.homedepot.com/Cleaning-Cleaning-Tools-Supplies/h_d1/N-5yc1vZ1xjvZbqqz/R-100200920/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053