We tend to make our knock-outs a few inches wider, when belting out openings into masonry. We then dab plasterboard to the uneven masonry jambs, affording us a nice straight, upright surface, onto which we fix the casings.
In your case, you'd be best off wedging in a vertical timber shutter and use bits of masonry, or re-build properly from the floor up. There may be room for masonry (deep bits) and foam, but you'd have to do it wisely. Wet it first and the foam will actually stabilise the masonry.
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