Hydrogen containment: you might want to look at topping up hydrogen along the way. Have a small compartment inside the gas bag with lithium shavings and a small servo that can release water into this compartment.

You'll get about 1/15-th of the lithium+water mass as hydrogen. So if you want to replenish 1 m^3 of hydrogen at STP (or around 1/8-th of the gas bag volume), you'll need around 600 grams of reagents.

It actually doesn't even have to be a servo. Just have a water container that slowly releases water through a capillary tube/wick into the gas bag, "moisturizing" the gas inside.

Lithium will then scavenge all the moisture from the gas inside the bag, releasing hydrogen in the process.