After drilling a hole to get things started, I inserted the saber saw blade into the hole, and got a good ways around the outline before running into other objects:
Next effort was to use a single blade hacksaw, but that didn't work too well -- for one thing, it wasn't going to make the turn sharp enough:
I had looked for a keyhole saw I thought I had before stealing the hacksaw from The Wife's tool box in the house. So, I looked again in the workshop when her saw didn't work, to no avail. Even checked Home Depot and they didn't have a keyhole saw, but they did have a cutter for the Dremel tool. That finished the job:
Used a belt sander and a rat-tailed file to round over the "bank" of the pond-to-be a little. At the top edge, the bank is to be trampled down a bit from the horse going to water there. Hard to tell from photo except for sanding dust on re-railer!
The plywood is 1/2", so I looked for something more like 3/8" to lower the pond bottom and secure from underneath the layout.
Most guys that have used two-part epoxy for "water" warn of sealing all leaks, so I used some left-over clear caulking. . . probably a little more than I needed. Is has a 7-14 day fully cured time, so I'm on hold for a while. Here's hoping that's enough caulking so it doesn't leak, AND the caulking doesn't get dissolved by the epoxy.