Do a web search for Webike Japan ( I haven't seen one for sale anywhere else) and look under items for the GSX-S.
I've just bought one of the original helmet locks from them -you know, the ones they used to fit to bikes as standard - You remove the left side pillion foot peg arm and the helmet holder goes there. The helmet holder is designed to fit with or without the pillion footrest as you wish.
Like you, I like to attach my lid to the bike but it has to be a proper job. The lock looks neat on the bike, comes with two keys and you don't have to carry chains or cables to attach your lid.