I'm probably going shopping for this tomorrow. Here's the concept:
Make a stand that holds 1-2 pieces of posterboard. The posterboard goes down the middle of the map so the two sides can't see each other.
It's a morning fight and a fog bank is obscuring everyone's view.
Both sides start wherever they want on their board edge.
When the first ship of either side crosses the wall, that's the simplistic cue that the morning sun has warmed the area up enough and burned off the fog enough so that everyone can at least see well enough. Just lift the wall out of the way. This will be even better with two card plotting. They'll still have one more turn of moving they can't change for an increased "oh sh****t!" factor if anyone suddenly finds themself very close.