"Modesty" wall

[RKB]

I need to build a "modesty" wall between a bath and a WC, something that screens the WC from the bath.
It needs to be about 4 foot high, and the width of the bath (3 foot?).
I planned to use a frame made out of 4x2, then cover it with tile backer board. However, although I can anchor the frame against the wall and the floor, I think it will still flex at the top unsupported corner.
Can anybody suggest how I can make this absolutely rigid?
Thank you

 

[DAZB]

I would be surprised if it will still flex by using 4 X 2 over such a short span but if you cut and fit a diagonal brace of 4 X 2 timber from the bottom corner near the wall to the top external corner and make sure it is a good tight fit this should work a treat. If not skin it with 12mm ply first then the tile backer.