Best material and method for boxing in behind toilet

last time I used mdf and glossed and sealed around edges. was ok until wee from my 4 year old penetrated it. Think it started getting under a screw cap. anyways it started to smell now and want to redo it. I would like an easy access panel to access the isolation valve but thinking of tiling to match the rest and to help stop wee marks etc.
is there a preferred material?

 

You could tile and use kiku fitting for concealed access.
http://www.screwfix.com/p/keku-fixings-pack-of-10/62708?_requestid=914410