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 2: Making the base
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
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.