Try making a blackhole base:
Take a starbase hull, put on an MC, repair bays, and TONS of the beefiest armor you can get.
If you have over 5000 hitpoints, and enough repair to fix that every turn, your base won't be harmed by the hole!
AI ships that venture nearby to attack will get sucked in and destroyed
The hardest part will be in completing a starbase in three turns. You need to bring lots of repair ships, and do a long retroseries up from an empty starbase hull.