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This is a great project - just one minor note...the "elbows" are actually "corner braces" or "angle brackets." I know it sounds nit-picky and I don't mean to be - but elbows are usually some kind of pipe/plumbing fitting. I was a confoosed and I know what I'm looking for- just thought a noob would appreciate the actual name if they were searching out parts somewhere besides Lowes.
Those tubes are actually used as concrete forms - for pouring a deck piers and the like (you fill the tube with concrete). If you go to the "big box" just ask them for "concrete form tube-cardboard" and you'll get what you need. "Sonotube" is one brand name, so is "Fast-Tube", another is "Quik-crete Form Tube". One caveat- there is a huge range in price - the really cheap ones are very thin and would not hold a fastener very well. Real Sonotube is about 1/2" thick and pretty pricey. Tip- sometimes foundation contractors or deck-builders will have chunks of the stuff left over from jobs and might let you have a chunk. Hope this helps.