I have gone to a necklace configuration for my octopus, but the second best solution for securing an octopus; in my opinion, is one of those fastex clips.
-You don't use any belts, rather you zip tie the one half (the one with the angled end) directly to the hose about 4 inches from the second stage. Alternatively tie with string etc. For the other end, you attach to your BC D-ring, but you tie it with a few sections of rubber band. This hold the octopus extremely securely, will never fall off and cant get jammed with silt like a bolt snap.
In an emergency, you press the fastex clip and it opens to free the octopus. If there is no time for that, you, or the panicked buddy, just grabs the reg and rips it off breaking the rubber bands. As long as you replace the bands once or twice a year, it is extremely secure and simple and if the rubber bands should break unintentionally, then you have a loose octo for one dive, not a huge problem for a rec. diver. Never happened to me actually. It really is better than any off the shelf octopus holder I have ever seen and it costs almost nothing to try.
Again I prefer the necklace.