Hot glue. See Will any pre-existing phone cases work for Librem 5? - #4 by Ylegreg for a little DIY-video linki 
I agree on the case problem. Fortunetely there is already are stl files available for DIY printing: Files 路 master 路 Librem 5 Case Creating Community / Librem 5 model 路 GitLab (I haven鈥檛 tested them - yet).