How to make a killbox?

[DespacitoRoyale]

Okay, so every time I start a new base, I always get the essentials up. A covered storage area, a kitchen, a basic barracks, and a (would-be) killbox. But every time I try to make a successful killbox, it always fails. What do you guys make a killbox out of, and with what dimensions, what traps, etc.?

Mine:
15L x 8W
Gun Turrets or EMRG Turrets (I play modded)
Sandbags or Shields
double-blocked walls outside to make sure they can't break their allies out.
Ignore roofing on top.

I build on a mountain, so that I really only need 1 killbox outside my base entrance. But what am I doing wrong?

[BugPowderDust]

You need to give a little more information- why are they failing?

You basically funnel the attackers into a single point of entry- i do this at max range of your turrets, and so they overlap fields of fire. Put a couple of turrets nearer and off to one side, so they will bee-line for those turrets first. Surround nearest turrets with sandbags or stone/steel chunks to slow them down, and/or traps

Surround this courtyard with a single wall, so if they break in rather than go through funnel, they are still entering killbox. Double wall rest of base so it makes this approach more desirable.

Build cover for your colonists to add their firepower in some safety- eg. W-S-W-S-W-S   where W= Wall and S= Sandbag.

Just some ideas, there are many ways to achieve this.

[Canute]

Some people are doing a long open walkway, so raiding need along time to move through.
Since it is an open entrance and open door to their internal base normal raide rwill path through it.
Then they start a fire at the end and build a wall or close the door at the entrance to trap them and maybe the heat will give them heartstrokes.

[TheMeInTeam]

There are lots of viable designs, ranging from heat traps to melee to the usual "forced no cover spot with concentrated fire, limiting how many raiders can be there at once".  These can all struggle vs infestation/drop pod raid/siege/sappers, so you will still need some micromanagement/control of drafted pawns to defend properly.

[Bozobub]

I prefer to manufacture and reinforce chokepoints, rather than build killboxes.

The silly thing I do, instead, is to network the ENTIRE open area of the map with closely overlapped turret "pillbox" (protected by sandbags) fields of fire, then have turrets backed by WSW construction, then the colony outer wall, then another turret/WSW line.  "Defense in depth", essentially.  It's ridiculously expensive and rather absurd in many ways, but also much fun to build and absolutely satisfying to watch as enemies pretty much evaporate into bloody mulch .  In fact, exactly this construction is pretty much my main goal for 5/6 of a given run !