I think it comes down to pure LoS now (correct me if I'm wrong)
So if you're standing directly behind an aegis defense line, you should have no problems claiming a cover save, as long as your model is partially covered.
Rmbr, it's all model's-eye-view now (unless anything is area terrain). Which is why Gretchins behind an aegis defense line is not gonna see anything over it, maybe unless it's trygon-sized.