The fog chillers currently on instructables are a variation of the "fog on the rocks" design, but have a few shortcomings that small changes can correct.
For less money we can construct a re-useable fog chiller that will cool more fog, requires less ice, allows for more fog to be produced, and is considerably easier to assemble.
If you're looking to build a fog chiller, this is the one you should build.
Step 1: Justification, design changes, theory, and a few little tips for avid chiller designfolk.
The other two are examples of the "fog on the rocks" design that has been popular with home haunters since 1995.
The "Super Cheap and Easy Fog Chiller" doesn't produce a lot of contact time between the fog and the cold, resulting in less cooling. I imagine that a great portion of the fog simply blows clean through, the second design forcing the fog to sink is a better one.
The Fog Chiller for $10 blasts the fog directly into the wall of the cooler box, this is bad, more on this in two paragraphs.
Both of the other two chillers use styrofoam cooling chests as their chilling box. This is a rather bad plan for a simple reason, the fog eats into it and becomes toxic. The chillers are not exactly reuseable, your fog becomes toxic, and I hate the squeaky sounds styrofoam makes.
So, changes? The important one is the "fog cage". This is a gap between the nozzle of the fog machine, and the chiller. This is important because for the fog to form properly, optimally, and in maximum abundance it must mix with a LOT of air immediately. If it comes into contact with a solid object, or is in an air starved environment you end up with less fog. Less fog means you need more fluid and and a more powerful machine. Costs go up, way up. The change in the amount produced through the use of a fog cage is very dramatic. Those who have already built the styrofoam chillers should give pointing their fog machine into their chiller from a distance so that they can get the benefits of the extra fog without the extra work of making an extra chiller.
Second change is simply switching from a styrofoam chiller to a regular ol' plastic tub. It doesn't react with the fog, it's tougher, and you can store some of your halloween stuff it when the day is done. You may think the ice would melt faster, but the primary source of heat isn't ambient, it's the really really hot fog you're blowing in one end of your chiller (duh). Also, in my tests the chiller in the photos keeps the ice for about 7 hours of use with an automatically cycling fog machine.