Grohe 35015 valve installed inside concrete block wall

Following the Grohe instructions gives no indication of how to keep the valve secure inside the block wall. It has to be recessed inside the concrete block wall to allow for finished tile dimension alignments. How do you fasten the valve to minimize movement. Thanks.

You might have to get creative. Maybe use some construction adhesive to anchor a 2x4 or something similar in the wall to then screw the valve to it.