Shower valve not flush with wall

[BobM7299]

HELP! I replaced the mixer valve in the shower. The old one was held flush with the wall with a scrap of wood. I've been unsuccessful in duplicating that same hack. Any other ideas on how to secure the valve so it doesn't move and stays flush with the wall?

 

[havasu]

What is on the back side of the valve? A drywall room, I hope?

 

[BobM7299]

Yes, but I didn't want to go that route if I could help it.
I can't believe that little block of wood was jammed in there for 30 years!

 

[Twowaxhack]

Cut the wall open and install some blocking.

You might could push/hold it into position and spray foam it