It common to use a "kneeboard"[1] to secure an EFB to the pilot's thigh. I'm surprised it's not standard practice and that a loose EFB is ever acceptable.
1 - https://www.67d.com/cdn/shop/files/KneeboardwithiPad11Pro-22...
1 - https://www.67d.com/cdn/shop/files/KneeboardwithiPad11Pro-22...
Further reading: https://airfactsjournal.com/2018/10/how-spatial-disorientati...