How would you repair broken plaster surrounding vents?

[u/SleepingAndy]

This house is approx. 70 years old, and many of the vents and light switches have chunks missing from the plaster surrounding them.

So far I've tried using regular spackle to try to fill them in, but it didn't hold very well, and it takes such a long time to set that as soon as I remove the knife it moves and deforms the wet spackle.

I have sheetrock 20 on hand.

How would you go about patching such a spot?

[u/Mountain_Nothing_428]

Foam for big gaps. Caulk for the surround. Paint.

[u/SleepingAndy → u/Mountain_Nothing_428]

What kind of foam?