I have always been brewing in plastics bottles (not very professional, but for me it works without problems) and so I thought of some better way of getting CO2 out of the bottle and no oxygen in than capping the bottle and letting it squeeze when there's high pressure.
The liquid-filled airlocks are fine but making them is a horror and also, I was used to let bottles in cellar for long time until it's finished and sometimes I found out that it dried...
So there's one primitive airlock that needs no care.
It works just on two facts:
1)There's still higher pressure in the bottle than outside (this works until yeast dies or runs out of sugar)
2)Relative molecular weight of CO2 is 43.99 g/mol. Air has 25.93 g/mol. Result of this is when you have CO2 in bottle, it won't mix with air until wind blows into the bottle (at least for some time, due to diffusion).
Step 1: Materials
OK, now what will we need:
Step 2: Making the Base
First, take the sand paper.
If the cap has some imprint on it, sand it down so the cap is flat.
In the middle of the cap, use the sand paper and make it rough. It should not be smooth.
Now we will drill three holes so that an M3 screw can be put in them.
One will be in middle and the other on opposite sides, place them near the inner cap circle.
Now, rip off anything overlapping off the holes.
Step 3: Placing Screws
Now, put M3 screws in the outter holes with head down.
From top, put pads on them and then nuts.
Tighten until screws get partially inside the plastic (this will deform the plastic a bit).
This will ensure they will be air-tight.
Look at photos.
Step 4: Making Spring Holder and Placing Spring
Now, cut a piece of metal. The shape of it is up to you, but rectangle is easiest.
Its length should be about the same as cap's diameter.
Make it wide enough so it won't break when you make 3 holes into it.
Use rasp to make edges and corners round (for safety).
Now drill into it 2 holes of M3 size in the same distance as the screws are placed in cap.
Between them, make another hole. Its size will vary, because you need to use screw big enough to hold the spring.
Now, put a short screw in the middle hole.
Put pad and nut into the spring and screw it together with spring holder.
You can see it on photos.
Now just put the spring holder on screws and put nuts on them.
Step 5: Making the "valve"
Now, cut a rectangle out of rubber. It should be some rubber that doesn't bend very easily.
I used pipe sealing rubber.
Its width should be bigger than diameter of hole in cap.
Make it longer than its width.
Put some oil on the top of cap around the hole.
It should be some light oil with no smell, non-toxic. I would like to suggest you to use silicone oil.
(It is often sold as spray, very good for this use)
Put also a bit on the bottom of rubber.
Put the rubber between the hole and spring. It should cover the hole and spring should be applying some force onto it.
Use the nuts on screws to adjust the pressing force of spring.
It might be good to use some anti-corrosive or at least any protective paint on heads of the screws.
The rest which will not be inside the bottle doesn't need any paint, the silicone oil will be enough.
Good luck in brewing!