@FirstCavApache64, why do you say that when you already know that the bulk of the roots are towards the bottom of the bag. Roots don't want to grow against gravity they know which way is down. Agreed there's no set amount of soil because that depends on the plant and the substrate. When you say 'nobody' else can see it, you are just referencing half a dozen people on a single thread. All the best ideas are because nobody saw the obvious. They laughed at Apple laptops when they moved keyboards near the screen instead of near the user. But it was such an obvious move that once it was pointed out, it became standard.
Take the Eazy Pyramids, they are a solid substrate that can be made in any shape, when the shape is not dictated by what is easy, but instead what works best, all their moulds are narrower at the top and wider at the bottom. They could make instead of a pyramid, a cylinder like a grow bag, but they do not do that. And the reason is obvious.
Actually the velcro idea is better, you'd just need it for the neck of the bag. Well you wouldn't need it but it would make it a bit easier, the zipper was just if a bag was made to be easily reusable, but it is a bit of overkill.