How can I safely store highly outgassing self-pressurizing chemicals?
Bottles for 30% or stronger $\ce{H2O2}$ usually have vented caps to prevent sudden out-gassing, splashing and boil-over that you experienced, as well as to prevent bursting the container. The vent may be as simple as thinned spot, which may be scored or slitted, or it may have a polythelene, PTFE or other sponge or "paper" separator that helps retain liquids while allowing gas to escape.



In either case, I would not suggest storing a mix of $\ce{H2O2}$ and $\ce{HCl}$ for more than a few hours for a few reasons.


As gases evolve, the solution becomes weaker and less effective an etchant.
Stored on a shelf, the gases will corrode the shelving surprisingly quickly. You would not want shelving with caustic or toxic chemicals to collapse.
If gases given off are other than $\ce{O2}$, you probably don't want to breathe them for long.


Ferric chloride is a slow etchant at room temperature, but gentle warming speeds etching greatly (PDF). At 54 °C, $\ce{FeCl3}$ removes ~35 um of copper per minute, so standard PC boards ("0.5 oz" - "1 oz") should be etched in ~1 - 2 minutes.

The biggest nuisance I find using $\ce{FeCl3}$ is that it tends to stain containers, tables and hands, but Iron Out and similar bisulfite or hydrosulfite stain removers can be used on cleanup.