The premade rubber ones are no good. They are WAY too long (unless you're a yetti or something), and even then, the reg can and does pop out.
The backup regs needs to stay in place and be where you need it, when you need it. No if ands or buts about it. That video shows a way that can and does allow the reg to pop out of the bungee. Even if its on there tight, it can work its way out and then you've not meeting the above requirements for a backup reg. I used to dive with it this way, I thought it was super secure, then one day, it wasn't. Lesson learned.
The better solution is to put a simple overhand knot into each end of the bungee and then secure it under the mouthpiece ziptie, using pliers to tighten the ziptie to the max. This absolutely prevents the bungee from coming out.
The bungee needs to be just long enough for you to get the reg into your mouth. And longer and it dangles too much. I prefer mine up pretty tight, with just enough slack so the bungee isn't under tension when its in my mouth (if it were even a little bit shorter, it would be under tension). This keeps it up and out of the silt when moving through restrictions, prevents freeflows when in high flow/ scootering, and allows me to turn the mouthpiece forward when I'm scootin'.