I usually go with huge, 30+ kton PDCs with a fleet scanning suite (ship, FAC and missile sensors) ~20 AMM launchers, frakkdeep magazines and around 100 ASM box launcher. Also, very thick armor and barracks.
Something like this, from my current game:
Argonev class Missile Defence Base 35,000 tons 335 Crew 4786 BP TCS 700 TH 0 EM 0
Armour 18-95 Sensors 1/630 Damage Control Rating 10 PPV 95
Intended Deployment Time: 36 months Spare Berths 1
Troop Capacity: 5 Battalions Magazine 1970
Benson-Gough Aerospace S-5 Box Launcher (100) Missile Size 5 Hangar Reload 37.5 minutes MF Reload 6.2 hours
Newman & Robertson Armaments Company S-1 AMM Launcher (20) Missile Size 1 Rate of Fire 10
Atkins Heavy Industries Missile Fire Control FC67-R1 (1) Range 67.2m km Resolution 1
Sanders Electronics Missile Fire Control FC189-R100 (4) Range 189.0m km Resolution 100
S-5 Sapphire ASM (100) Speed: 28,800 km/s End: 38.1m Range: 65.9m km WH: 4 Size: 5 TH: 115/69/34
S-1 Sentinel AMM (1470) Speed: 28,800 km/s End: 3.6m Range: 6.2m km WH: 1 Size: 1 TH: 172/103/51
Fletcher Aerospace Strikecraft-Band ASS MR112-R20 (1) GPS 6300 Range 112.7m km Resolution 20
Fletcher Aerospace Ship Band ASS MR504-R100 (1) GPS 63000 Range 504.0m km Resolution 100
Fletcher Aerospace Missile-Band ASS MR33-R1 (1) GPS 420 Range 33.6m km MCR 3.7m km Resolution 1
Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s
This design is classed as a Planetary Defence Centre and can be pre-fabricated in 14 section