I needed a twin holder for Ravensburger's Tiptoi (see https://de.wikipedia.org/wiki/Tiptoi for more in German), and found https://www.thingiverse.com/thing:3473229 by Andre Weigelt pretty nice. I added cut-outs for screws and a hexagonal one to save plastic. As shown on the picture, it is suggested to print upside-down with brim.
PS: The two holes are not ideally placed for screws, but I initially wanted to use hooks before I decided to go for mounting tape (as you see in my make).
The author remixed this model.