DESCRIPTION
Ever since Distant Stars was announced, the devs provided details on how to create your own clusters. I've been waiting for an intrepid modder to do exactly that, and it seems I'm that modder. 

This mod adds a cluster of stars located outside the galaxy that can be traversed via several wormholes. You'll find some goodies inside. The cluster will be generated at the second day of the game and takes a few seconds to generate, similar to opening the L-Gate, so expect a little lag as it generates. 

While there are several clusters available, though only one will spawn in-game:

Hexagon Cluster
Steed Cluster
Atronach Cluster
Apprentice Cluster

Clusters still being worked on:

Big Dipper Cluster

NOT achievement-compatible, because it's pretty overpowered. 

Does NOT need the DLC Distant Stars, but it is compatible with it. L-Cluster will not be overlapped. 

Save-game compatible, expect lag on the second day after you load your save. However AI has a better time surveying if you start a new game. If a cluster already exists, a new one isn't likely to spawn,

Cross-compatible with most mods, as I only added files. Might not be compatible with other mods that add extragalactic clusters if their cluster uses the same coordinates as mine, but it looks like I'm the first to make one, so this isn't an issue.

POSTSCRIPT 1: Regarding Starting in the Cluster

I do not know if it is possible to start in a cluster at game start. It sounds like a long shot that will need a lot of workarounds and creative solutions. 

For now I can tell you how to start a colony in the cluster using a lot of workarounds. 

Start a new game.
wait for the cluster to spawn.
Open up the terminal.
type "observe" and go to the cluster.
go to an inhabited world and then switch back to your country with "play 0 " 
make sure the planet window is open and then type in "own" in the terminal
a. If there are no habitable worlds, just click any world and type "planet_class pc_tropical" or something. 
b. If the planet is above size 25, fix it with clicking the planet and typing "planet_size 25" or something. 
c. Add deposits with clicking the planet and typing "effect reset_planet=yes" then go back to #6.
cheat in just enough energy to move all your pops to the new world. 
Execute this script in the console to set up your new capital:
 effect add_district = district_city add_district = district_city add_district = district_city add_district = district_farming add_district = district_farming add_district = district_mining add_district = district_mining add_district = district_generator add_district = district_generator add_building = building_capital add_building = building_research_lab_1 add_building = building_foundry_1 add_building = building_factory_1
Results may vary. 
Dismantle your old starbase and all your ships (you cant move them into the cluster after all.)
Undo all cheats (like debugtooltip) alternatively, you could save and exit.
Tada! You technically have a start in the cluster!

You can see how difficult it is to manually move your capital to the cluster. I imagine that is how I would go about starting in the cluster but I have little idea how to write this down in Stellaris event code language (which is what governs starting civics, which I assume the a cluster start world use as a trigger.) 

Well good luck with that!

POSTSCRIPT 2: Regarding Multiple Clusters

I wrote the mod in such a way that only one cluster spawns at a given game. Though there is technically no real reason why multiple clusters shouldn't spawn except for clusters using same coordinates. Items with "⇔" have the same coordinates and should NOT be spawned together. 

Use "event extragalactic_cluster. to spawn in the cluster. Save before spawning in another cluster (just to be sure,) and do NOT spawn in a cluster that has already spawned.

Hexagon Cluster  Apprentice Cluster

extragalactic_cluster.1  extragalactic_cluster.4

Steed Cluster

extragalactic_cluster.2

Atronach Cluster

extragalactic_cluster.3

POSTSCRIPT 3: Got a cluster or constellation you want to add?

Suggest a cluster or constellation to be added to this mod! Please limit the number of systems to 15 however. Please make sure the constellation or cluster has constellation lines and aren't just one line or simple shapes. Real and fictional constellations are welcome!

Change log 2018-12-13:

Put in addtional edits kindly suggested by FLSoz. 
Added in some script in the description you can copy and paste to help set up a capital in the cluster.

Change log 2018-12-12:

Added the code kindly suggested by FLSoz that definitively prevents an already existing cluster from spawning again, even if triggered by console command. As a result, I have also removed the part of the description that warns about the double spawn bug. I have also written it in such a way that clusters with similar coordinates will not spawn if the other already exists. 

Change log 2018-12-11:

Made the mod compatible with 2.2. (Fixed all primitives, pre-sapients, ringworlds, etc.)
Moved the Hexagon cluster down galactic south because I really should have done that ages ago.

Change log 2018-11-11:

Player can now manually spawn in multiple clusters through console commands. 

Change log 2018-11-06:

Added Apprentice Cluster and Atronach Cluster.

Change log 2018-10-31:

Added a global flag for cluster spawning and modified parsing of the spawn requirements.
Added the Steed Cluster.
modified instructions for starting in the cluster.

REQUIREMENTS

Stellaris

Full game

FORUM LINKS