Are you talking about the telescopic Dahon stem clamp/handle post that goes up on a down?
https://foldingbike.biz/epages/7665e...dlepost_Uppers
I swapped the handle post on my helios and I find it much stabler than the telescopic version. The T version is monobock ~ £60 + £40 for the syntace kit.
I did it on the jetstream too however, the jetsteam handle post is monoblock but it has a top handle bar clamp (like the telescopic version. I used a litepro tube (25.4mm from memory) in the post clamp and the syntace braces joint the tube to the handlebar...
you could do the same. or alternatively, get a cheap aluminium straight flat bar, put it on your handle post, fit the syntace braces and cut to size... then, add you handle bar the the syntace brace and pop 4 end-caps to finish it off nicely
https://www.decathlon.co.uk/p/flat-h...523372-2635386