We bring my daughters makala dolphin places when I know its going to be hot or stay in the car. I would be fine bringing any of my $300 ukes to the beach, I got them to enjoy them. The car thing however often forces me to use the dolphin because I don't want to have to bring the uke inside every place with me. Also my daughter always winds up stealign the dolphin back, as it is hers.
I would say get a piece of foam stuff it in there, then pop on the blue painters tape and you have two layers of protection, heck even toilet paper could be stuffed in it. For some reason the idea of the spare plug put in there bothers me, metal on metal and if moisture got in you couldn't see I could see the metal sticking together and maybe being hard to pull out? I like the idea of something that sucks up moisture being put in the hole, like cotton balls or something that naturally would take the moisture in even if its invisible to my eye.