I did the same thing, Backed off the trailer, lit it up 30 seconds later water pouring from the whole where forgot to put in the temp sender in the front of the intake. Before I left the house, I'd run it for a short while on the trailer, with no water, and decided it was good to go.
I tried twisting a sock in the hole which actually worked until 3000rpms. Finally found a spare 1/2 pipe boat plug, saved the trip, won't make that mistake again. :hammerhea