Forget that.. what they need to do is quadrupple field ack. Take away single 20mm emplacements and add batteries of sandbags inside the same pad. Thus instead of 1 gun you have 4, all firing at the same target. And increase their rate of fire.
The sandbags mean that if a plane makes a cannon attack at one gun, and hits it, the other guns are not touched. Meanwhile, to MAKE said attack you have to brave 4 separate guns all shooting at you. A small bomb will take them all out... A rocket might take 1-2 out. But intensify the hail of gunfire from a field (at least at low-level).
In real life an entire SQUADRON of tempests made a super-fast low-level attack at a LW airfield. Only 2 made it out alive, and one of those was so damaged it crashed shortly afterward.
The dinky "Kindergarten" ack we have is the reason people dive lever bombers. There's nothing to deter them. Basically they can get away with it. So just make them DIE much faster and they'll stop.