Works as of patch 0.24.4.13 0 BETA

Honestly, you probably won't need one of these. The game is incredibly easy in its current state even on very hard mode. After 3 playthroughs I decided to try building a killbox my 4th playthrough, and so far it mostly helps save on repair resources.

If you still want to learn how to make one for fun, or so you can relax more, then here you go.
Will definitely let you take on double or more the infected you usually struggle with, though.

You will need to wall off your entire base but one entrance.
"Bait" Squads will be necessary, but only on the inside of your base. They will not come into dangerous situations.
By moving squads inside and out of buildings where the Infected can see them, the Infected will go all the way around to the kill box entrance.

Do any of this wrong, and your kill box might not work at all.

You can also put gates at the sides of your base if you want to make it easier to get out.
Barbed wires are necessary for slowing down the Infected when they enter your killbox.
Put as many as you feel comfortable with.
But they also slow down your own people so try to have gates as options for your tiny civilians.
Gates might need to be empty of workers for the baiting to work.

Watch the in depth video below for a better analysis and guide.

In Depth Video:
Straight to the point, and not a waste of your precious time.
(I don't make ad revenue anyway.)

Potential one survivor strategy? Who knows?
Either way, its entirely optional, and the game is single player, so I'm not judging anyone.
Play how you want to. Have fun.