Of course turret workbench is a suggestion, because why just light and heavy turret??

So the turret form which is gonna be upgrade-able should have a 1) sensor head (eyes); 2) weapon; 3) optional search tool; 4) Platform; 5) Armor; ( 6) Ammo feed )

Upgrades which is found in the wasteland or bought could for example be:

Spotlight for number 3. Or infrared sensor for number 1 + infrared search light for number 3.

Weapon (2) can also be upgraded to suppressed and faster. etc.

Turrets can be upgraded partly at the weapon bench and partly at the armor bench in settlements.