Quote fromAs an idea for the portal room...
Don't just survive in the Nether, Tame it!.
Find a suitable cavern/arch bit and block off all "entrances" (like 200 blocks big) with glass until the volume is enclosed. To keep ghasts out, place glass blocks in every 5x5x5 chunk. Now terraform it! Replace red dirt with grass and plant trees (no red dirt, no spawning pigmen?). Doing this to a large volume would be incredible, with just metres away on the other side of the glass, ghasts and hordes of zombie pigmen roam.
Doing this to every safehouse would be impossible, but it would be an awesome place for the main portal.
Well what if the ghasts start bombing the glass sections. They shoot from a far, not exactly sure of it's range though.
They'll just bust it to useless shattered disappearing thingy.