Hm, interesting concept.
I went and tried, sure enough, can't seem to build an engineless design with fighter factories. THAT BEING SAID... why bother with engineless? Engines use material yes, but they also use only ~10 tons and make the craft manuverable: Consider the following design:
Here is a beam targeting system for fighters, note the excellent tracking speed and short range for PD.
PD Sattelite class Fighter 385 tons 8 Crew 83.5 BP TCS 7.7 TH 28.7 EM 0
10649 km/s Armour 1-4 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 0 PPV 6
Annual Failure Rate: 77% IFR: 1.1% Maint Capacity 0 MSP Max Repair 36 MSP Est Time: 0 Years
Supercharged FTR engine (1) Power 82.5 Fuel Use 6000% Signature 28.875 Armour 0 Exp 50%
Gauss PD Gun 1 (1x3) Range 30,000km TS: 10649 km/s Accuracy Modifier 100% RM 4 ROF 5 1 1 1 0 0 0 0 0 0 0
PD Defense Sattelite Fire Control (1) Max Range: 30,000 km TS: 25000 km/s 67 33 0 0 0 0 0 0 0 0
This design is classed as a Fighter for production, combat and maintenance purposes
These just used a stock gun. It would probably be trivial to come up with a basic turret or similar design. So its got the fast trackign speed from fighter-only beam PD control, sits inside planetary active sensor range, and has an actual speed of zero but base speed of 10k, it should be able to turn and shoot at missiles within point blank range with little problem.
Do fighters outside a carrier but in orbit of a planet use planetary maitenence facilties? If yes, heres your final PD net.
Transfer to other planets by carrier. Let me know if it works. My fighter manufactories are shut down at present.