File, sand and otherwise flatten one side of the two bands. The two bands should absolutely flat and both on the same plane so the are no gaps when they are place on a flat piece of 20 gauge SS sheet.
To flatten pieces like this I use a woodworking sand paper mounted on a hard flat surface. I use 180 grit PSA (Pressure Sensitive Adhesive) coated paper on a hard maple block that fits in my bench pin slot.

I placed all the hard solder paillions on the surface of the sheet inside the form. With the charcoal block on a turntable the out side of the piece was heated while being rotated. The result was to draw the solder from the inside to the outside assuring a good joint. Watching for the flash of solder around the outside of the part tells you when the job is done.

BTW the parts are coated with a boric acid and borax in alcohol solution prior to each soldering step to help guard against fire scale.