The Lumiquest FXtra does NOT fit the 580EXII comfortably, it's a bit of a reach. I have one and never use it. It's also somewhat flimsy and the gel holder on my separated from the storage part (though admittedly it was because the thing was a gift and my sister's dog had a bit of time with it before it got to me :P but it doesn't seem particularly strong anyways). I'd either go with the Cinch-Strap (or Speed Strap from Honl) route and then velcro on your gels, else maybe some tape.