Safety/efficiency of a makeshift aluminium can heatsink?
Answering your questions in somewhat the same order as you asked:

1) If the device package was running too hot to touch while it was exposed to the open air, it definitely needs better heatsinking than it currently has.

2) By spreading the heat over a much larger surface area that can then radiate to the plastic enclosure, you are helping the situation.  As you noted: the plastic case now feels much warmer than it did before you did your modification.  That means that you are helping the heat get out of the device.

You may want to add even more surface area to the heatsink if you can do so safely.

3) So long as your aluminum can't short to anything internally and so long as nothing external can touch the heatsink, it should be perfectly safe.

From my point of view, the mod that you did is worthwhile and should greatly increase the service life of your power supply.  Good Job!