Soldato
I can't tell if you're joking but I don't care. That's what I'm using, thanks for the idea. I was going to completely patch it up properly but **** it.
Not joking, it's great for stuff like this. Fill the void as much as possible and then plaster it up with some one coat plaster, sorted. I'd dab some PVA mixed with water over the foam and the edge of the plasterboard before adding the plaster, just so the differing suction backgrounds dont cause it to crack.