Only room for one cage...

[OakLeaf]

zip ties? sure they're ugly, but most people use them to attach cadence sensors, computer cables, etc.

[Zen]

It ain't pretty but it does the job-
http://www.twofishunlimited.com/bike.html

[Mr. Bloom]

That's the one I have. As long as the velcro is tight, it's just fine.

[Eden]

Hose clamps - I used them to put a bottle on my old Marin, as it also had no bolts on the seat tube.

[VeloVT]

I was going to say hose clamps too. They are **slightly** less ugly than zip ties (in my opinion, anyway). They work quite well too.

[uforgot]

I've used the Minoura water bottle cage adapter and it works really well. The pics don't show it, but you can put it on the tube where your bottle normally goes also. That's what I did with my mixte.

[Bad JuJu]

You didn't say whether you also use a Camelbak, but it's something you might consider. If you had a Camelbak for water, then one water bottle--for electrolyte replacement drink, perhaps--might be enough.