Shower controls cover

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FIL hasn't showered for a long while and we are desperate to sort this out. MIL finally told us the reason, which is that he messes with the controls whilst she is trying to shower him. Is there a cover we could buy to stop him touching the controls ?

You could do tape and an A4 plastic wallet. Improvisation? I'm tired, and seem to be doing a lot of improvising with plastic wallets at the moment! Not meant as flippant as this will no doubt read. Sorry for that. But it might work? Just enough to stop him being able to burn / get it too cold?