Praise be to Allah the Lord of the Worlds. Works that facilitate usurious transactions of conventional banks are unlawful. However, your (Asker) creating engineering designs for those banks doesn`t directly facilitate their usurious transactions. Therefore, you aren`t to blame for that work, and the money you earn from it is lawful.
Nonetheless, we recommend that you avoid working for those banks since whoever gives up something for the sake of Allah, Allah will substitute him with something better. And Allah the Almighty knows best.