We develop a declarative DSL - ConstraintFlow - that can be used to specify Abstract Interpretation-based DNN certifiers. In ConstraintFlow, programmers can easily define various existing and new abstract domains and transformers, all within just a few 10 s of Lines of Code as opposed to 1000 s of LOCs of existing libraries. We provide lightweight automatic verification, which can be used to ensure the over-approximation-based soundness of the certifier code written in ConstraintFlow for arbitrary (but bounded) DNN architectures. Using this automated verification procedure, for the first time, we can verify the soundness of state-of-the-art DNN certifiers for arbitrary DNN architectures, all within a few minutes.