Different model, but similar arrangement. Mine between the shower and entry door cabinet. You can see where they installed a panel, and where the valve actually was located.
I reached in, found the valve, determined where I needed to cut a new hole, and used a hand saw to cut a square I thought would be large enough.
I ordered an exact replacement valve (made of clear plastic), but when I received it I could see tiny stress (?) cracks all around it. So I sent it back and eliminated the valve. And use an external backflow preventer.
I bought some trim material and made a picture frame around the piece I cut out and it looks like it was factory made for it.
