Insert every connector from the inner-side of the case and the ring washers and nuts from the outer-side. The ring-washers will avoid damaging the case surface when you tighten the nuts. A wrench or pair of pliers can be useful for tightening the nuts.
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Insert the jack connectors from inside the box and secure them with the wasters and nuts from the inside. The washers will keep the nut from damaging the case.