My current setup has a water bottle in a cage on the seat tube
my frame bag is correspondingly smaller as a result
i use a long / wide MYOG top tube bag with a capacity of about a litre.
The top tube bag is ok in the dry but in rain the stuff inside invariably gets soaking
wheras the frame bag keeps things much drier.
so i am thinking about having a full frame bag and mounting the water bottle on the top tube
has anyone done this ? / how / pro's and con's
any other ideas welcome. The aim is to keep stuff i need ready access to drier
Cheers Mark

I do sometimes on the Stooge. My frame bag has an additional strap that runs over the top to secure a bottle - never lost it yet but the twin top tubes on the Stooge probably help hold it in place.

I mount a bottle on the top tube at the seatpost end using this system http://vincero-design.com/product-category/bottles/ it has been totally secure and locating the bottle on the mount is dead easy. For me the main benefit was not risking disembowelling myself on a bottle cage if involuntarily getting off, the downside is the cost for the magnet mount, the bottles aren't mega expensive but the mount is.

I just use one of my stem cells to hold a mineral water bottle ( one with a ‘spot cap’ ) with the added bonus of keeping it out of the sheep poo

Monkey clip and monkey cage - Works well.

I slip a 1.5 litre pop bottle over it if there is too much mire.