Anyone seen any websites that give detailed instruction on the correct way to replace/install a shower tray? Mine has cracked, had to get a fibreglass one made but it's not quite flat. This means when you stand on it at flexes a little. This will not last long I fear so I need to glue it to a plywood sheet or something to stop it moving about before I fit it back in again. I was planning to use some spare mastic strips which I think would work? I just don't know how it's "supposed" to be done and can't find any websites or books on it either! Anyone care to point me in the right direction?