By the looks of the pic doesn't appear there ever was a cross brace.....You might be able to contact AGA or someone and see if they have replacement top trim of some sort or you could remove the trim yourself and clean all the silicone off of that area really well and get glass cut and eurobrace the tank which would handle the bowing.....Honestly 1/2 glass isn't skimpy for a tank that size....but should definitely cross brace, it, but IMHO shouldn't be bowing....What's the height of the tank?