I never know what to do with my hands when I sing unaccompanied.
Which is one of the reasons that I took up playing mandolin (along with the pleasure of playing per se) Now, on the rare occasion I sing in public, I can rest my hands on the mando if I choose, while I sing unaccompanied.
Otherwise, I've tried holding them in front of me, behind me,etc.
The last time I sang, my wife remarked that it was a good idea that I'd had, to hook my thumbs in my back pockets, as it opened up my chest. I'd just done that unconsciously.
By the way, I'm sure that I've seen some videos of Norma Waterson "conducting" whilst singing - I always thought her hand movements looked quite graceful.