I have built a solar panel which is used to add electrodes to $\ce{H2O}$ and $\ce{NaCl}$, evolving $\ce{H2}$ and $\ce{O2}$.
How can I safely store the hydrogen in the long-term?
Hydrogen can't and won't explode as long as it is kept separate from oxygen, unless you're storing it under high pressure. So as long as your hydrogen container can't break or leak under all sorts of environmental hazards (heat, fire, dropping, collision, degradation, etc.), you should be fine. Essentially, this question is more about risk management to consider all the possible scenarios of a potential leak.
Ultimately it depends on how and where you will store your hydrogen container, and the potential dangers of that environment. So storing hydrogen under low pressure within an argon-filled safe would be a lot safer than storing it under high pressure in a mobile oxygen tent.