Focusing simulation in FLIP tank on one area/hiding boundaries of tank?

[bentway23]

I have a FLIP tank with some action going on. It's just someone diving, so the part of the tank that requires a lot of computation is small, but it's an underwater camera, so with a small tank you see the edges of the simulation. Is there a way to either focus the simmable area in a tank/make particle separation dramatically smaller in one area, to accommodate a huge tank with not much going on so the render times aren't crippling? I've seen tutorials on using lo-res sims to drive hi-res, but that is sort of the opposite of what I'm looking for. I've also seen some endless ocean tuts, but since what I need is an underwater dive, and the breaking of the surface (and the water still beforehand), I haven't found a way to make that work.

Thanks for any help!

[bentway23]

In case someone else chance's upon this--the redoubtable Tanto responded on the SideFX forum, thus (quoting without permission, but with much gratitude):

 

In the Regions tab of your Particle Fluid Surface SOP, you can specify a bounding box, flatten the surface near the edges of the box, and then extend the edges using Pad Bounds and Extrude Polygons.