If you want to practice efficiently without a lot of up and down, which can be murder on your ears, find an old SMB and put a (very) small hole in the top. It'll hold gas long enough to to perform normally, but can be hauled back down and the process repeated without you having to go to the surface.
Once you get over the hump, it's just something to "stay current" on.
p.s. Even a full bag will not stand up on its own. You have to hang on it. If it's a 10' bag, you'll have to hang pretty hard -- or you can put enough gas to fill the first 6' and then hang on it. Not quite as hard as you would if it were full, but probably harder than if it were a completely full 6' bag to start with -- if that makes sense. But, that's splitting hairs. The goal is to have enough sticking out to be visible. How much that is depends on conditions.
Once you get over the hump, it's just something to "stay current" on.
p.s. Even a full bag will not stand up on its own. You have to hang on it. If it's a 10' bag, you'll have to hang pretty hard -- or you can put enough gas to fill the first 6' and then hang on it. Not quite as hard as you would if it were full, but probably harder than if it were a completely full 6' bag to start with -- if that makes sense. But, that's splitting hairs. The goal is to have enough sticking out to be visible. How much that is depends on conditions.