| Poorly fitting plywood used in window frame to mount evaporative cooler.
| Plywood is warping away from siding. I intend to try some screws to bend
| it back. If unsuccessful need to fill in space to keep water leaking in
| at warped spot out of house.

I still can't picture it, but if rain is getting into
a vertical gap I wonder if maybe a strip of aluminum
flashing would help.

| I've tried caulk but it doesn't work in
| this situation for some reason. Keeps opening up gap.

That may be from expansion and contraction
of the plywood. If it has to resist water and
needs to stick well then maybe the polyurethane
construction adhesive would be best. (Not the
regular stuff. That's been downgraded to be
fairly useless for much of anything. I don't know
why. Maybe it was a VOC issue.)

The spray foam is for insulation. It wouldn't work
well to stop water getting through.