For the majority of my commuting time, I've kept at least 1 pannier for essentials (a small crescent wrench, a multi-tool and a spare tube (which reminds me that I'll need tire levers too)). However, sometimes I'm just running a quick errand that doesn't require cargo or I'm out for a fitness ride so I just need the essentials (and a Clif bar in my jersey. They're awesome!), so I got a small under-seat bag from my LBS. It's big enough for everything (even my HTC Evo) but there's really not enough room for a spare tube.

Does anyone know of a creative way to carry a spare tube without a pannier or under-seat bag? I've seen some people tuck it between their seat and seat rails but that won't work for mine. I've seen others some how loop it between the seat rails and where the seat-stays meet the seat tube but I'm still kinda lost.

Any help is greatly appreciated. I've added a picture of the installed bag below if it's of any help.