For one of my projects I needed to bend a piece of flat Aluminium profile into a circle.
The aluminium sheet comes between the two parts which are then clamped onto the Aluminium, thus forming it.
From the radius I calculated how much Aluminium profile I needed an cut it to that size. I would recommend that you mark your length with a little bit of padding on both sides, because the end pieces do not bend very well.
If you leave some padding you can bend a full circle and than simultaneously cut off both paddings.