I would suggest that the length offered by Zemlin for the 600 f4E FL VR should work with your 600 f4G VR. So, looking at the Zemlin site, the 600 f4E FL VR replacement hood is the 206mm long version - the first one shown here:
www.zemlinphoto.com
Unfortunately, it looks as though there is no similar length for the 600 f4G VR. So, you either get the shorter version which is 158mm long (which is probably fine) or it might be worth sending an email to Karl and ask him what the diameter of the two piece collapsible hoods are so that either may fit in your backpack.
www.zemlinphoto.com