I build some SPA "Hummel" by BF in 15mm. They are no "Nashorn" but the real thing used the same chassis - the "Geschützwagen III/IV: chassis of the Panzer IV, engine and drive train of the Panzer III - which means: Same hull, same side and back armour around the crew compartment. The "Hummel" had different armour at the front of the crew compartment. To cut a long story short: It wasn't that easy you would expect from a BF kit to get all nice in straight. Perhaps it just isn't easy to do a proper tooling because of the real thing. But as you said: In the end it look wheely good.