(Please ignore the quality of the hand drawn diagrams, they will be replaced soon)
One advantage of living in the Finland or New Zealand is that home distilling doesn't carry huge fines or jail time. In New Zealand home distillation is completely legal, and in Finland you only really get in trouble if authorities find out you are selling the alcohol you distill. Because of the legality, there has been essentially a "crowdsourced" alcohol improvement research project going on for decades.
It's said that there is nothing new in the world, and in this case that saying holds true. People have been rediscovering some of the ancient techniques pioneered by people hundreds if not thousands of years ago. There are writings by the Chinese and the Romans dealing with the improvement of distilled alcohol that detail the method in this blog. More recently, around 1500 the Germans published a novel on the subject, a full 200 years before anybody put whiskey in a barrel in Scotland.
Basically, people in New Zealand and Finland have rediscovered one of the oldest and most effective ways to improve the quality of distilled alcohol. Full sized barrels inherently utilize this process by default when they let gas slowly seep in and out of the seams of the barrel when it isn't 100% full. The good news is that using this ancient method, you can achieve a better quality improvement in much less time than traditional barrel aging can. (side note: is it really traditional if these other techniques predate it by over 1000 years?)
First some background. When alcohol is distilled, the "heads" come out of the still first because they are lighter than the regular ethanol. Next comes the base spirit, and at the end of the cycle, the tails come through. They come last because they are heavier than the base spirit (ethanol). Both the heads and the tails are undesirable, and can even make you go blind if there is enough of them. On the other hand, without any distillate that contains heads and tails, you get vodka which has zero flavor. A balance must be made between purity and flavor by the master distiller. The barrel char deals with the heavier tails over time, and the angels share deals with the lighter heads over time.
In the diagram above, you can see that the gasses can escape through numerous places in the cross section of the barrel. The evaporated volatile organics and heads sit higher in the air pocket than the evaporated base spirit (ethanol) since they are lighter. Since the barrel has openings at all levels within the air pocket, both the bad things (volatile organics and heads up high) and the good things (base spirit) escape over time. This is known as the angels share. Ideally, you wouldn't want the good stuff to be escaping, only the bad stuff. Making matters worse, there are more slots between staves for the good stuff to escape than the bad stuff in a barrel which means that with barrel aging you are loosing more good stuff than bad stuff over time.
Now, as you can see in the diagram above, you can engineer a better angels share than the professional producers can when they use barrels. The 5 important variables are the flow rate through the mouth of the bottle, the ratio of liquid volume to air volume, the ratio of surface area (where the air meets the liquid) to liquid volume, the evaporation rate of the heads and volatile organics, and the height of the column.
Since the air can only escape at the top, and the bad stuff is naturally at the top, only the bad stuff will escape as long as the flow rate is less than the evaporation rate of the bad stuff. Having less surface area will cause the evaporation rate to be lower. Since gasses tend to mix very easily, having a taller column will allow more space for the lighter bad stuff to separate from the heavier good stuff. Whillett Pot Still bottles seem to be ideal for using this technique. Just make sure that the fill level is low enough that the full wide radius of the bottles lower area is utilized to create a large surface area for evaporation / oxidation.
If you simply had the cap off of the bottle, all of the gasses would escape constantly, and the heads and volatile organics sometimes take a while to off-gass into the air. The flow rate needs to be low enough that the bad stuff can evaporate and separate itself to the top of the column before the rest of the air column escapes.
I recently ran a blind tasting where I applied this old world angels share method to one bottle, and had it tasted side by side with a bottle of identical bourbon from the exact same barrel. Whiskey #1 was the standard bottle, and Whiskey #2 was the same bourbon that had the old world angels share method applied for 6 months. The rule of thumb for this method is that 6 months gives you 6 years of quality improvement (from traditional professional barrel aging). 85% of participants picked Whiskey #2, and 100% of participants who rated their whiskey experience level above a 3 picked Whiskey #2. The full results are below. In a recent blind tasting by a expert, he thought that Whiskey #2 has the smoothness of a typical 90 proof bourbon, despite it being cask strength at 129 proof.
All you need is 3 coffee filters and a rubber band. Place the coffee filters over the open top of the bottle, and place the rubber band around it to keep air from escaping. Ideally it should not be disturbed since that causes the "bad stuff" and the "good stuff" to separate less. Feel free to experiment with various flow rate limiters other than coffee filters like a thin piece of oak on a mason jar, and/or the number of coffee filters.
(Edit: the proof went down roughly 3 points for the 6 month version.)