Inside windows

We have just purchased somewhere and whilst decorating the top of the window inner bit has collapsed revealing a metal like frame.
i have brushed the remaining cement away and want to put in a piece of plasterboard.
I obviously have nothing to screw it to - so if i tried to dot and dab it with some bonding plaster would it take???
There are some holes in the metal grill, but it is mostly cement so nothing really for the bonding plaster to get into.
Any advice ?

epanding foam
cut a piese of plaster board to size wet the back of it wet the area to be covered spray in some expanding foam bare in mind it will expand up to 500% depending on brand then hold plaster board in place with some tape leave over night if possible then trim any foam that as escaped
bare in mind very sticky avoid contact and cover the floor just incase if any gets onto window frame or glass leave over night untill it sets then remove

thanks much appreciated......