I run a normal rucksack these days, and put the fluid in a side compartment.
Been toying with the Idea of getting a second Carrier, and having one if each side to balance it up, and for Longer rides.
Currently I drop a 1 litre plastic squash bottle into the other side.
Main problem I have with these things is, it's to easy to drink it all and find you've totally ran out miles from anywhere or anything, back when I had a bottle on the bike it seemed to last ages.
Spare tube, multi-tool pump spare tube puncture repair kit spare set of disc pads, never had to walk home in 14 years except for the two times my brake blocks wore through my rim, but that was in pre disc days. Also I can carry all those items in a seat pack and so can just use a water bottle if I want, and theres no checking up to make sure that I,ve got everything pre ride.
What an arsehole, I use a LOBO with two tubes, a topeak hummer, a mountain mammoth pump and a paclite, wether Im doing a Merida 50 K or going for a two mile cycle, and with well maintained bikes I never have a problem.
I should have a mech hanger, especially for my up coming long distance stuff, keep forgetting, Must order 1 as I've bust a few of these on previous bikes.