Hi forty, looks like you got it all figured out! Yes like Karsten answered above it can be a good idea to look at your surfaces using Edit > Show pts to see the control point structure of the full underlying surface - it's that full surface structure (shrunken down to contain the active area, but still using the same grid structure) that will be used for the flow, any sort of notches out of the side that are only trim curves won't really have any impact on the flow, as compared to having the full underlying surface itself shaped how you need.

So if you need an irregular boundary you'll want to use something like Loft or Sweep, etc... to construct a full surface that has that structure built into the surface itself - the type of surface you get with Construct > Planar will be a big extended rectangle that then has trim curves on it.

Just wanted to make sure it was clear, it sounds like you have it figured out already though.

- Michael