Perhaps you could visit your piercer for proper measurements or purchase a pair of calipers for home use so you'll know exactly which dimensions you need to order. The piercer could also readjust your Anatometal retainer by widening the space between the legs.
I too have issues finding comfortable septum jewelry, so I experimented with quite a few jewelry options to see what worked best for me. You could certainly contact the folks at BAF (or Glasswear Studios) to see if they could find a custom piece for you.
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I just read your other thread concerning the removal of your jewelry. If you keep changing jewelry, you're going to doom the piercing to excessive amounts of scarring and painful healing. Even flipping a retainer up and down can greatly hinder healing. You really shouldn't even change septum jewelry until the second or third month of healing. Please find a reputable piercer immediately to help you, or just leave the retainer/barbell down during the healing process.