Aluminum Fence posts set into concrete
The forms have a top. You temporarily attach something to the post that holds it at the proper height when sitting on the top of the form, and you set a couple of temporary wooden stakes at 90 degrees to each other from the form that hold diagonal braces that hold the post plumb, and leave all of those in place until the concrete sets. With aluminum posts this might involve a lot of clamps to hold the post to the braces and supports.