How about a base inside a black hole? Enemy ships would be destroyed by the hole if they attacked.
All you have to do is construct a base, and retrofit it before it falls into the hole.
To survive the hole, you just need 5000 hitpoints worth of armor. Then add some repair facilities to fix that much every turn. Now you have a base that nobody can touch!
If you can manage 5000 armor on a baseship, even better! You can lurk in a black hole system, and then jump out and disable engines on an enemy fleet, then drift back in to be repaired by the base
