The mic cover muff kept on falling off my Peltor earmuffs I use at work. The fragile hook type holder had snapped two of the four legs and couldn't hold on to the mic cover anymore. So I printed this little replacement. Now the mic cover stays in place even when I play air-guitar, so I call that a successful repair.
EDIT 09 Sep 2019: So I dropped my earmuffs AGAIN... The printed part cracked into several small fragments. So I redesigned it a little with added support beams to the top. It worked way better than the first one.