Alcohol bubbling

Letting the air pass through a strong alcoholic solution, as vodka or absynthe.
Alcohol is known to be hostile to a lot of organisms: keep in mind that surgeon sanitize their hands with products based on alcohol.
If you do set up a good turbolent flow without losing too much contact time, you should manage to reduce in great part the microbial count in your airstream.

Geometry of the apparatus, on a budget

The idea is getting plenty of curves, so air has to go through multiple alcoholic phases and optimally a final washing water phase. Getting one more washing water phase in the middle would be ideal.
Stainless steel piping fittings or traps (also called siphons) would be the ideal choice: coupling multiple U fittings will give you this solution for less than 25$, and it will hold to sanitation standards higher than the food industry has.

Water Absorption

In technical terms, it'd be called "absorption airstream contamination": ditching sanitizers in favour of strong alcoholic solutions will already make the air more-than-FDA safe, but if you really have an issue with air containing a small part of alcohol, you can place another fitting or trap where clean water will rest.
This process is used in the pharmaceutical industry, for a safety regulation framework of reference. Using distilled water in this curve would be ideal: it tends to have a very low starting microbial count, and the alcohol it will absorb (thus removing it from the air stream).

If you do choose this solution, I can estimate the amount of water and\or the maximum air flow to get a fixed concentration of alcohol, or if you choose the bubbling solution I can relate the number of curves.

Even when they tell you otherwise, traces of chemicals always linger by the very nature of chemical reactions: the smaller the concentration, the slower the reaction. After a while, it slows so much it practically stops.

Live easy, very small traces can be safely neglected both by taste and health, and I don't mean figuratively: under a certain concentration, there is no evidence that a substance can alter the equilibrium of your body. How low that certain concentration is, however, varies by substance, hence my offer.

Limits

Only a few alcohol resistant strands would pass, and they'd have to be more easily dispersed in air than water or liquid phases: competitive growth should hold them back, however.

What's the problem with foil? When I cold crash, I seal the fermenter either with a solid stopper or foil. Since fermentation is done there's no need to do anything else. AAMOF, you want to stop air from getting into the fermenter, whether it has microbes or not. I would not recommend using anything porous.

Northern brewer has them in starter supplies. That said star san becomes yeast nutrient in contact with wort or beer. This from Charlie Tally interview on BN in '07.