Elfa "bracket post" cap replica, for 420mm brackets. https://elfa.com/en/assortment/garage-and-storage/bracketpost-478170

It prints with the same final height as the original, so you can mix/match them in your Elfa system. I modified the top profile from semi-circle to flat to help support heavy objects.

Rather large, won't fit on most printers until you slice the object 220mm from the rear (end without the standing cylinder) and print as 2 pieces. At the 220mm section you'll find a buried 6mm blind hole on each side. Was experimenting with pinning and gluing the halves together, but found they grip the brackets plenty well so I use them as 2 piece items.

Not done with this yet. Want to add another interior wall to lock the front half into the bracket. Maybe overkill, it functions just fine as-is.

Pictures below: blue part is 3D printed, grey part is original Elfa.