What size top box for a fzs600?

[stevierst]

I'm thinking of getting a custom rack made and the guy reckons it will take more weight than I am able to put on it... but I've seen a lot of posts on here about racks sold as accessories for the bike failing. People seem to swear by custom mare racks but what makes them intrinsically stronger? Aren't both kinds just metal and welds?

It depends on a lot of different things. I used to make racks for forum members, and I still have one on my sons Fazer, my Thundercat and the FZ1. I used quite heavy duty steel for the mounts, and sometimes extra support is needed which isn't provided on 'off the peg' racks.
You wouldn't believe how much vibration and bounce occurs with a rack, and they overhang quite a bit too. (Laws of levers kicks in) So 5kg in the top box can be considerably more strain on the mounting to the bike, especially when its bouncing around. Something has to eventually give, normally at the weakest point which is just to the rear of the rear mounting bolt where the grab handle mount is.
And you REALLY don't want one to break on the move!!! It could have some nasty consequences.  :eek