I always thought it would be a better idea to use a ton of fighter-sized missile platforms so that building them was easy and losing them was no loss. If you have an ordinance transfer station hub or spaceport on the planet you don't even need since the planet can reload them. You'd need a separate Active Sensor platform to spot, but that can be fighter-sized as well.
EDIT:
I threw together something real quick. Take a look:
Missile Platform
Missile Platform class Missile Base 493 tons 10 Crew 244 BP TCS 10 TH 0 EM 0
1 km/s Armour 1-5 Shields 0-0 HTK 3 Sensors 0/0/0/0 DCR 0 PPV 5.4
Maint Life 0 Years MSP 0 AFR 98% IFR 1.4% 1YR 33 5YR 489 Max Repair 126 MSP
Magazine 36
Commander Control Rating 1
Intended Deployment Time: 0.3 days Morale Check Required
Size 6.00 Missile Pod (6) Missile Size: 6 Hangar Reload 122 minutes MF Reload 20 hours
Small ASM Missile Fire Control (1) Range 110.3m km Resolution 20
Compact ECCM-3 (1) ECM 30
This design is classed as a Fighter for production, combat and planetary interaction
Sensor Platform
Sensor Platform class Missile Base 421 tons 16 Crew 370.8 BP TCS 8 TH 0 EM 0
1 km/s Armour 1-5 Shields 0-0 HTK 2 Sensors 0/0/0/0 DCR 0 PPV 0
Maint Life 0 Years MSP 0 AFR 84% IFR 1.2% 1YR 161 5YR 2,419 Max Repair 315 MSP
Commander Control Rating 1
Intended Deployment Time: 0.3 days Morale Check Required
Medium Mid-Range Active Sensor Suite (1) GPS 18000 Range 149.1m km Resolution 100
ECM 40
This design is classed as a Fighter for production, combat and planetary interaction
Point Defense Platform
Point Defense Platform class Missile Base 485 tons 20 Crew 281 BP TCS 10 TH 0 EM 0
1 km/s Armour 1-5 Shields 0-0 HTK 6 Sensors 0/0/0/0 DCR 0 PPV 5.91
Maint Life 0 Years MSP 0 AFR 96% IFR 1.3% 1YR 37 5YR 549 Max Repair 126 MSP
Commander Control Rating 1
Intended Deployment Time: 0.3 days Morale Check Required
Single Gauss Cannon 25.00 Turret (3x5) Range 50,000km TS: 25000 km/s Power 0-0 RM 50,000 km ROF 5
Gauss PD Fire Control (1) Max Range: 57,600 km TS: 25,000 km/s 83 65 48 31 13 0 0 0 0 0
Fighter Active Sensor Suite (1) GPS 4 Range 4.5m km MCR 408.7k km Resolution 1
Compact ECCM-3 (1) ECM 30
This design is classed as a Fighter for production, combat and planetary interaction
EDITEDIT:
I realize now that you want this platform to mostly deal with enemy missiles. In that case I'd still suggest fighter-sized platforms armed with gauss cannons AND/OR these AMM platforms.
AMM Platform
AMM Platform class Orbital Defence Platform (P) 493 tons 15 Crew 188.9 BP TCS 10 TH 0 EM 0
1 km/s Armour 1-5 Shields 0-0 HTK 3 Sensors 0/0/0/0 DCR 0 PPV 4.8
Maint Life 0 Years MSP 0 AFR 98% IFR 1.4% 1YR 15 5YR 218 Max Repair 63 MSP
Magazine 45
Commander Control Rating 1
Intended Deployment Time: 0.3 days Morale Check Required
Size 1 Missile Launcher (8) Missile Size: 1 Rate of Fire 25
AMM Fire Control (1) Range 28.7m km Resolution 1
Fighter Active Sensor Suite (1) GPS 4 Range 4.5m km MCR 408.7k km Resolution 1
Compact ECCM-3 (1) ECM 10
This design is classed as a Fighter for production, combat and planetary interaction