if you feel u are capable use 2" x 2" timber and build a box frame around your concealed cistern , you must make sure the toilet can still be connected after you apply your 18mm ply, 10mm wedi or 10mm backerboard etc., and the depth of a tile and adhesive!!!!......cut holes and slots in relevent places and you should end up with a box with two pipes protruding........put top rear and side battens first, then square off them...........good luck!!!!