Hello MIB.
I use 2mm dia brass rod or 1/16 in old money. Put it against the tin and bend it from the centre outwards not from one end, and do it equally at both sides at the same time. You should be able to do it with the rod cold as its only thin.
Bend it past the centre line and it will spring back a bit then bend the sides for the tower sides when the dia is right. If it is a little out you can adjust it when you put the uprights on and solder them to the ring. Go slowly and use the full 12 onch rod don't use tube.
Hope this is of use if not I dont think you can buy it pre-done anywhere
