I think the OP has it about right. A 6x8 shelter is NOT something you're going to use for more than shelter for an hour or so. I wouldn't even bother with the jack. I've worked tornado SAR and in the very, very unlikely event you'd be trapped in the shelter, the rescue folks are going to be there in an hour or so anyway. A cellphone...preferably a smartphone with weather/radar app, or at least a WX radio; lights, water, maybe a potty. That's all you're going to need in tornado country.

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Rick, W0FG
CTR2 68-72