I've not seen ones that hollow if I'm honest. The plaster on the wall looks thick enough to take a 25mm metal box without chasing the brick?
That wall is in a state. It's crying out to have the plaster removed, holes filled with mortar and pieces or brick, and then dry lined with plasterboard!
Yes this is in the uk the plaster is not too thick but that bit of brick as soon as you chisel to it. Its hollow behind and its not very good condition
Enlarge the hole (if necessary) to take a lightweight block.
Get one, make some mortar, fill in the hole.
When set, cut the recess for the box.
Maybe think where else you'd like sockets, and do all of the wall patching in one go - if you have a suspended floor you could install conduit and empty boxes with blanking plates - you wouldn't have to do all the electrical work at the same time.
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