Also... supposedly ribbon and powder are more flammable than a solid magnesium rod. This makes sense from a surface area perspective.
Secondly, it sounds like experiments have demonstrated that the effects are due to hormesis, and not direct antioxidant activity. (The latter could still be important, but mediated by downstream effects instead of electron donation from H2 itself.) This 2012 metastudy notes: "Two enigmas, however, remain to be solved. First, no dose-response effect is observed. Rodents and humans are able to take a small amount of hydrogen by drinking hydrogen-rich water, but marked effects are observed. Second, intestinal bacteria in humans and rodents produce a large amount of hydrogen, but an addition of a small amount of hydrogen exhibits marked effects." The answer to the second enigma is probably that hydrogen produced by gut bacteria is not sufficiently well dissolved in liquid to have any meaningful effect. But the answer to the first enigma sounds like a classic example of hormesis at work.
What to take from this? First of all, buy rods, not powder or ribbon. Secondly, don't go to great lengths to maximize your dose at any one time; rather, take a normal glass of the stuff just as you would with a glass of water, a few times per day. Otherwise, the extra H2 concentration may do you little or no additional good, but the excess magnesium could result in brain fog, kidney stones, or other issues. (The brain fog is reversible, at least.) Clearly the test tube method could help out here. My problem with that method, though, is that it's unclear to me how to remove the test tube in order to recharge the acid level without causing sanitation problems or evicting too much hydrogen from the water in the process. (Maybe one only needs to recharge once per full soda bottle consumed?)
BTW that metastudy claims that the maximum possible H2 concentration in water at room temperature is only 1.6 ppm. I suspect this conclusion is the result of reliance upon erroneous lab work, but it does at least call into question our method of measurement. Nevertheless, it sounds like the issue is moot if the hormesis hypothesis is accurate. And it wouldn't surprise me, because H2 is incredibly tiny, not unlike the radium atoms in radium water which, in tiny doses, made people healthier. (Don't go drinking radium water, though, unless you want bone cancer!)