Heat Transfer - flow

Greetings.
I have a Heat transfer modelled, which I'd like to couple to a fluid laminar flow.
I would like to give the flow inlet and outlet conditions on imaginary surfaces on either side of the geometry (as given in the figure) . However, I understood a box selection doesn't work.

Kindly, help me in defining an imaginary surface to give fluid inlet and outlet.
If defining a wall geometry is inevitable, How should I be modelling it, and what material properties shall be assigned to it.

Thanks.

-Sai Phani Kiran