I am in the process of replacing an old leaking recessed shower mixer valve mounted in a stud partition. I have removed the old valve and small area of tiles cut away water damaged and mouldy plaster board around the existing opening as far back as the remaining tiles. I now plan to install a surface mounted Grohe 1000 bar mixer using the existing pipework built in to the wall. The mixer is to be mounted on a bracket fixed in the existing recess with the pipes poking through the tiles.
The problem is how best to fill the hole left from the old valve housing and the extra cut away damaged plaster board. This hole is 20 cm wide by 40 cm high but may have to be 5-10 cm wider depending on state of plaster not yet uncovered. I propose to fix two vertical batons screwed to existing woodwork inside the opening an then covering these with a piece of plywood fitted flush with the existing plasterboard/plastered surface and then tile over this leaving suitable holes for the backet to poke through.
Is there a better alternative? Should I use plasterboard rather than plywood? If so how? Any solution needs to be fairly minimal in extent since I don't have any spare tiles and they are no longer available. I am conscious that there could be slight differential movement around the edges of the repaired portion and the rest of the wall which could affect the integrity of the tiling and grouting.
All ideas gratefully received.