• PLA works fine. Mine have been in use for over a year, with no issues.
• Walls: 4 (for strength)
• Make sure you have not enabled outer walls to be printed first, as the overhangs will likely fail if you have.
• Print with brim for better adhesion.
• Print with opening surface downwards, and you won't need support.
• Do not print with the hole/mounting surface on bed, as the layer lines will then compromise the integrity.

If your printer and filament settings are calibrated properly, this print will have no issues.

The finished print can be turned upside down as well, but I recommend installing it as shown in photo, for maximum strength.

Will fit most types of spraybottles.

One 3mm screw will be enough to hold a regular, full bottle. Second hole is for positioning, though it's optional. Don't tighten too much, or you risk breaking the plastic.