I doubt there is a free gallon-sized-space anywhere in my car's engine bay or in any modern engine bay for that matter.

There is space in most trucks. My Jeep Wrangler has enough room to stick two 1-gallon water bottles in the engine compartment -- one on either side -- and actually be drivable that way.

I prefer smaller bottles, either .5L or .75L. I can stick one or two on the dash to defrost, a couple more in the engine compartment if I want. If I run out of gas I can shove one under my coat to defrost using body heat. Smaller is better with the last method.