I've got two bikes and several backpacks, so I have found the easiest way to carry the essentials like a spare tube, multitool, puncture kit etc. is to use a small saddlebag with a bracket on both bikes.
The bag ripped open at the weekend and I lost a load of kit while riding, so rather than replace it with something similar, I was wondering if there's a water bottle with a full diameter neck that can be used for carrying stuff.
This would be easy to swap between bikes and would keep the CoG a bit lower as well.

I've got one. Not sure where it came from; CTC magazine I think. Fits in standard cage. I'm not using it now so it's yours if you want it. Could cost you zillions of beer tokens!!!! Mail me.

Dunno, but I like the idea.

Get yourself down the patent office if nobody makes one already.

Trek have already done it:

http://store.trekbikes.com/jump.jsp?itemID=1125&itemType=PRODUCT&path=1%2C2%2C15%2C487%2C488&iProductID=1125&bShopOnline=1

I think Giant used to do one.. not sure these days though.
If you've got a Thomson stem bag, then you can put a bottle cage in it, burn some holes for the bolts and screw it on. Worked way better than a saddle bag for me. I guess you would need 2 though!

theres also these: http://www.probikekit.com/advsearch.php?AQUERY=storage+bottle

these come in 500 size and 750 . very good they are too 😉
http://cgi.ebay.co.uk/PRO-BICYCLE-TOOL-STORAGE-BOTTLE-500ML-12991_W0QQitemZ390125251613QQcmdZViewItemQQptZUK_SportGoods_CyclAcces_RL?hash=item5ad546ec1d

These from BBB

I use the 750ml Pro storage one – very handy.

Lost two inner tubes and god knows how my mobile and house keys didn't fall out.

Deffo considering that Trek softshell bottle carrier.

Thanks for the suggestions and thanks for the offer, Sir neil.
I like the look of the Trek one, no need to tip everything out to get to the stuff at the bottom.
I think I'll go for one of these or maybe the bigger version. As long as the neck's big enough to take a rolled up tube, it should do the job.

I used to use a plastic 1 litre mixer bottle cut around the middle on my commuter. Free, totally waterproof and easy to pull apart. I've recently uprgraded to a Robinsons Apple and Blackcurrant diluting juice bottle as the ridges around the circumference snap together nicely and it sits quite snug in the bottle cage. Big enough to hold a repair kit, spare tube, cloth, latex gloves, spare chain links, splitter, tyre levers, you get the idea.