I made a model out of 4 basic modules layouts:
Airlock module - litle square
Base module - the main starting module.
Rectangle module - as in your schemates
Connector (or Triangle) module - it can connect 3 modules together.
Player would decide for himself where to put the airlock. He simply connects an airlock module to the desired module.
Using those 4 basic module shapes player has almost infinite possibilities how to connect them together.
If you use Connector module to connect with every other module you will see that the modules connect to each other together and make a circle.
P.S
Iam sorry - I actually made some serious schemates. Each one different, and many of them. I would gladly post them on forum, but Iam currently traveling to Italy
P.S.2
You should also see this:
http://janek.kozicki.pl/asia_bergamo_2009/asia_bergamo_2009-18.jpg