Got around to finishing the flash hider today. The barrel is about 15mm OD, while the flash hider is 14mm ID.
About 30 minutes on the dremel, and it fit right over the barrel. The grub screw is actually the same threading as the AK front sight grub screw, so I can have the grub screw threaded into the flash hider and barrel at the same time. Worked out great.
Very happy with how it turned out.