Good thread. These kind of mods would be easy when I was employed and had access to a full machine shop. I have a choice:
- I can make an all-new seatpost as you have done, which might require reducing the outside diameter as you have done to fit in the frame bushing, which is a bad idea on two fronts: a) Your reduction in size constitutes a stress concentation at the highest point of moment, and b) surface finish must be extremely smooth for similar reason, to reduce "notch sensitivity", or
- I can go to a smaller standard size tubing and use multiple bushings at the frame to step down the size, or
- I can take an existing 33.9 seatpost, take the top hardware off somehow (or if constant wall thickness, cut it off and invert the post), and use multiple bushings at the top, with a slot cut for clamping, to adapt to a more standard size of suspension seatpost.

I have a long 33.9 seatpost that has a huge hole worn in the side about mid-length if I recall, the person I bought the bike from was a trucker and the seatpost rubbed where he had it stashed. I may experiment with that, seeing if I cut it at the hole, is there a standard length suspension post long enough to make up the difference, and do the last option above.

But I don't bike off-road. I should first try to ride any bike with a suspension post, to see if I bob on the seat due to pedaling, and if significant improvement in road ride.

I think I might get more benefits from a suspension stem, but that is currently impossible as I have an early Dahon Speed with a fixed-height steel stem, with the handlebar clamp welded integral with the stem tube. I'd have to measure the diameter to see if a modern stem would clamp to the outside of the tube.