A friend loaded up a motor at Plaster Blaster (2 weeks ago) just before they closed the range. He just wrapped electrical tape over the aft closure to keep anything sparky from getting into the motor and packed it away. This morning he took the tape off and flew it and it worked fine. I think I would have loosened the closures to take pressure off of the o-rings and then retightened them right before flying.
A year and a half ago, I had put together an F40 to put in the Moostang for the 1st annual TRF gathering at Whitakers. When it started to rain that first day, I didn't get the launch off and the next day was too busy getting my L2 attempt ready (shush Ken) to launch it. Well, I finally launch the thing (assembled F40 still in the back end) almost a year later. Worked just fine.