Nitpick: I don't consider Flak excessively fragile. It has the same HP (21k) and frontal surface area (3) of the Large Cannon, or an effective ratio of 7000 hp/frontal tile, beating the Laser family and approaching vertically stacked armor.
Of course, due to the volume/depth it's completely implausible to mount shields (and armor) behind it, as is the standard practice with cannons, but figure in its ability to shoot down incoming projectiles and they're reasonably survivable in sub 1.5m ship fights.