I'm sure there's an easy fix for this problem that I just don't know yet.

I'm an undergrad and currently constructing a tubular network for biological testing. I'm attempting to run flow simulation through the network. I've left the end of my inlet and outlet flow channels open (for 3D printing purposes). There is an open edge at each edge of the inlet and outlet, even after I've added chamfers to reduce flow turbulence. Creating lids or a surface fill still leaves me with the open edge, therefore a non watertight model. Any help on how to accommodate this for flow simulation would be greatly helpful. Unfortunately, I'm not at liberty to post my file, but I've attached a simple screen snip of the target area.

Thanks