If the weather is cool enough that I might need a jacket, it's probably cool enough that I won't need a second water bottle for a typical 20-30 mile ride. So I'll use the seat tube water bottle cage to hold my thin windbreaker. I stuff it into a quart sized ziplock baggie and jam it in the cage. That's worked fine for the past few years.

Thursday was unusually cool here in Texas, in the 60s (after temps in the 100s last week), with a chance of rain. I intended to ride 20 miles or so and took one 24 oz bottle, and stuffed the jacket into the second cage. Turns out I felt better than expected after warming up and did 40 miles. I needed the jacket for the final 10 miles as it was sprinkling and cooling down. Still had water in the first bottle when I got home.

I've stuffed that jacket in my jersey pocket, but I'd rather not. So far the ziplock bag and water bottle cage trick has worked for most rides.

I could have saved some trouble and worn a Pearl Izumi wind jersey I got last winter. With a good compression baselayer it's more aero and flaps in the wind less than a jacket, breathes better and doesn't get too sweaty. I've been caught in chilly light rain with it before and it was comfortable enough.