This is a design I am coming up with as an answer (and I am sure there are many possibilities) for D7's lasting for as long as they have...
Qa'Pla!:
Klingon AD7-H Ko'Dagh-Subclass
Direct Link:
https://s27.postimg.org/y9dbe17yr/Concept_Klingon_AD7_H_Variant_02.jpg
Qa'Pla!:
Klingon AD7-H Ko'Dagh-Subclass

Direct Link:
https://s27.postimg.org/y9dbe17yr/Concept_Klingon_AD7_H_Variant_02.jpg