I am increasing the height of a concrete block fence using timber battens attached vertically and then sheeting over the entire fence with hardieflex/durasheet 6mm. Battens at 600mm centres. New fence height will extend 800mm above existing. I am using 75mmx45mm battens. I am concerned the battens supporting the top section may twist or bend. Should I use something thicker say a 125mmx75mm every 2.4m to give extra support? Or maybe put noggins in the top section? I will use 75mm x45mm for the top capping.