My Brompton bike (monotube folder, 16" wheels) has this stock seat post
The stock seatpost is made of chrome plated steele and tends to slip down during the ride. I am thinking of replacing it with a custom machined aluminum seatpost with slightly rougher surface.
How should I go about spec'ing the custom seatpost? For example, what material? Any special treatment? What inner diameter Anything else?
Safety is important, weight is less important (though it will be nice to keep the weight reasonable). Also, the seatpost interface with Brompton Pentaclip sadle adapter so I need to preserve the shape of the top end (this excludes most off the shelf after market seatposts).