I use these religiously :
http://www.treefortbikes.com/product/333222372576/553/Dimension-Sidemount-Cage.htmlThey are cheap, sold in just about every LBS I've ever been in, and you can bend them to make them grip the bottle super tight.
Another great thing about them is that they are totally reversible, making them ideal for fork-mount.
I used them on a complete thru-ride of the AZT, and never lost a bottle, even in the thorny bushwhack. No extra strap required.
Camelbak Peloton bottles seem to be a good match, FWIW.
Scott