The local hardware stores actually sell chemicals in recycled beer bottles with plastic stoppers as lids. The bottle of hydrochloric acid I bought is clearly losing gas to diffusion through the plastic, or a seal that's not tight enough--it smells bad, and the beer bottle's metallic label is slowly corroding.
How can I best seal this bottle? Some ideas are:
Try to mold a stopper from silicone. Use a (synthetic) cork, whatever's used for homebrewing. Line the stopper with a few layers of thin polyethylene or polypropylene (from consumer materials, such as air-filled plastic bags for packing).
Or could I limit the diffusion by putting a plastic bag around it? This is tough to answer experimentally (without a lab) because resisting physical degradation isn't the same as resisting diffusion.