insulate empty void on return brick work to hang plasterboard

[baker332]

Hi all,

My return brick works is slim up to my bi fold to prevent a step in.

This proves a little difficult there is a wind post in the corner.

How would i insulate this void but for then plasterboard to be dot and dab over the top. There is nothing for the plasterboard to hang onto. I can add a cavity closer but it won't have a strong enough fix surely?

Thanks