I think I own the same pair of shorts that des mentioned. I like them. Also, you can try out the fuel belt "wedge". You wear it in the small of your back.
http://www.fuelbelt.com/s.nl/it.A/id.1213/.f
I've worn it in all of my marathons. It works pretty well. I'd recommend doing a practice run with it first, though. I've used this for all my marathons (3 x Octane GU mixed with water), along with water stops. I don't know how well 5 GUs would mix with it but you could do X in the flask and X in the side pockets if need be.