Meshing round orifice on physical wall
 

I'm trying to generate a 3-D grid to model a chamber that has a small round orifice on one straight wall. So far I can generate the round hole with a 5-subdomain for one wall, but in order to build a block I am required to generate another 5-subdomain on the opposite wall. But then I have to connect the two opposing walls, and this results in a "tube" or cylinder connecting the two orifices. This is not what I need. Any help will be appreciated.