As there was no io holder yet perfect for my bathroom and application, I made this neat wall mounted station for my Oral-B io 8, it's charger and my toothpaste and brushheads, using two other models on thingiverse (links below).
The plug of the charging base fits through the slot. I mounted the holder with double sided bathroom tape (Tesa bathroom mounting tape) and just in case thats not enough I supported it with a bracket. I glued the charger in place and ran the cable down the back for a super clean look. Under my sink is a timer so the toothbrush only charges 20 minutes a day, rather than permanently being plugged in.
I have now used it about a year and absolutely love it. The only downside is some crusting up of toothpaste remains on the head holder, so I made a version with ventilation holes that might be better.
Depending on your printer and fillament you might need to enable supports for the bridge in the back where the cable will exit the charging base.
These are the models I used as a template:
https://www.thingiverse.com/thing:5164782
https://www.thingiverse.com/thing:4678744
The author remixed this model.
Removed parts of each one to add to my holder.