Drop by your local building supply store and purchase two EMT conduit hangers that will fit your tube. They come in overlapping sizes between 1/2 inch and 2 inches. Here is a photo of a 1/2 inch hanger holding a flashlight.
Just bolt both hangers to your bottle cage and then clamp the hanger around the frame tube like the one holding the flashlight above. I've seen them used to attach tire pumps, stoves, and stove fuel bottles and even large sections of PVC plumbing pipe to bike frames.
Be sure to wrap some electrical tape around the tube where you plan on clamping them to protect the paint.
Here is another photo of a hanger being used to hold a dynamo headlight built inside a 1 inch PVC pipe coupler.
