Just a simple holder for 3" and even some 5" quads for backpacks without straps or needing an additional mount. Uses x4 M3 screws and x4 M3 nyloc nuts to attach. Your screw size will vary depending on the thickness of the backpack/material but through my thin one it needed about a 10mm screw. The holes are tight, you might want to drill with a 1/8" drill bit before to make it easier.
You will need supports if you lay them flat or on the bottom. At least with the bottom (the way it is placed) no one sees versus turning it on it's side and having a bit less support. Your choice.
You can also choose to remove some material to speed the print up but I went with more material and 20% infill for a nice strong holder.
M3x10mm or so works best with a M3 locknut through a thin backpack such as this.
Inside of the bag
The bump on the front is generic to hold a quad in place but is made to fit the Tooth Fairy. (please excuse the dirt in the screw heads I just noticed lol)