Yes, it will be fine. A Faraday shield doesn't need to be grounded. It just needs to be conductive and fully encompass the radio. From what I read, you don't need to put it in a Faraday cage so long as there's no long wires connected to it. The wavelengths in an EMP from a nuclear bomb are fairly long, so an antenna of significant length is required to collect that energy. An HT without the antenna probably doesn't need to be in a metal box. On the other hand, there's talk of existence of purpose built EMP weapons that are more effective at frying electronics, so to be totally prepared you do want things in a box. Note that the box's lid should make electrical contact along most of its edge. It doesn't need to be 100% tight, but sections that don't make contact form a slot antenna.