Originally Posted by
BassNotBass
I have yet to find a cost effective solution so I go the swamp-yankee route and use whatever is on hand. I've gone several different routes on various bikes. On some I welded two of the longest seatposts I could find in my metal scrap bin and braced it internally with tight fitting angle iron. With others I've used 3/4" Schedule40 black pipe, chucked it in the lathe and turned it down and welded on an appropriate sized reducer for the saddle mounting hardware. They're heavy but they don't bend and I'm not a weight-weenie anyway... I just go out and ride these things.
IIRC the longest chromoly or aluminum seat post I could find was 450mm but I'd want something closer to 500mm. You would think I could find something appropriate from the folding bike community but the cruiser frames tubing diameter is too narrow. My Brompton has a 530mm long seat post and easily handles my weight but then the diameter is 31.6mm.
What is the smallest diameter you have to deal with? Mine is 7/8" for long posts and 13/16" for shorties. I was surprised at the low prices of aluminum solid rod and tubing when I first began looking into making my own posts. I bought a 42" scrap for about $12.00 and made three posts. Perhaps 7075 would work for you?