We've tried numerous things - firstly the multi zone foam which wasn't that great - we then bought a cushion style overlay from deals direct - getting better but I found that the smaller section does seem to be softer (and slightly lower) than the large section of mattress which seems to make you really feel the zipper. This weekend i cut down the multizone foam, covering only the small section of the mattress and the zipper and then putting the cushion overlay back on. It does appear to make the zipper feel less noticeable. Will be away for 2 weeks over easter and see how this works....! 
