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I've had a go at my own (see my previous post). I'm trying it first without a gas struct. Even without one, for my purposes, being able to lower and raise, one-handed, without repositioning, is an improvement. I realised my bike has two slots for the seat clamp, so I could run two bolts and slots instead of one. In order to get the first slot straight I laid the seat post down on a flat and even surface, pushed a piece of box section along it and scribed a line down it. I then used a sharpened steal bolt, to deepen the first slot and create a second slot, by threading it into the modified seat clamp. At first I tried using a relatively thick seat clamp, but the aluminium was to soft to hold a thread. Having to start again was unfortunate as I'd already made two grooves on the seat post. As there are two bolts and slots the spacing needs to match up. So I ended up using the bolt and nut ends of the now ruined aluminium clamp with three layers of 28mm waste pipe solvent welded together for thickness. To create a thread I recessed a couple of nuts on the inside of the pipe (hence the pipe thickness). I used a couple of sharpened M4 brass bolts, with the hope that brass might be softer than the aluminium seat post. I've shortened the bolts to a length that includes a pair of washers each, that way if the bolts wear I can remove a washer to compensate. I left a bit of thinner pipe sticking up above to stop the actual seat clamp from rotating.

Thanks for making the photos available again. Helps to show how you avoided the post rotating while its being adjusted. I'd be tempted to try drilling the hole on the opposite, already slotted, side of the seat tube. Not sure if it'd work out, but it'd be less noticeable if I ever resold my frame. I found this page because I was thinking of using a shim to provide a slot inside the tube and adding pins or a ridge to the post in order to avoid rotation. That way I'm not taking material off the post. Would be reliant on something like silicone to avoid the shim rotating though. Was thinking of this kind of diy job on my jump bike. I ride to the start of trails and getting the seat straight (and up to the right height) each time is a pain. I've got a downhill hub and drivetrain on it, but only really so I can ride to the start of lines/trails. I've got a couple of droppers including a Brand-X, great value and no-nonsense. Maybe I should just pay the £100 for an externally routed Brand-x, only 125mm adjustment though and another cable for the remote (it being a jump bike and all).