I had intended to make a new seatpost for my bike because the original one wouldn't go high enough. My bike's frame is almost 10 sizes too small for me, but I've been able to manage by getting a stem extender and raising the saddle as high as it would go. So high, that the minimum insertion line was exposed, and it still felt too low.
I was planning on finding some tube to turn one down from until I found some conspicuously shaped tubing for free on the streets. They were about 4 feet long 1" tubes that were reduced to 7/8" on the ends. 7/8" just happens to be a standard dimension for saddle attachment hardware, and 1" happens to be a standard seatpost dimension. The problem is, my seatpost is a 26.4mm (actual dimension 1.037" or 26.3mm) which meant I would need to make a shim.