Denis,

More coverage = more better. Pardon my poor English, but that's easy to remember. You want enough thermal mass to counteract cooling. More water will have more thermal mass.

As a practical consideration, 3/4 up the side should be plenty, but if all you have is a shallow dish to hold water, use it. Anything will help. The problem with the shallow dish is that it won't hold a lot of water, and the water it does hold may freeze quickly, depending on the temperature.

I've seen butter/margarine type tubs used to good effect. Place the tub on a square of closed cell foam to insulate it from the ground, put in the canister, add water, then fire up the stove. I've seen a "cozy" made out of closed cell foam used with the plastic tub, which also helps keep the water from freezing.

I haven't tried it, but I've heard talk of using a Ziploc bag to hold water to put the canister in. Can't get much lighter or more compact than that!

HJ