I haven’t checked the specs – BUT I had four or five of these literally melt / become twisted from just sitting in the back of my truck for a couple of hours in the sun/heat. Perhaps just use a metal box ...
Please check your local code to determine what is required for your application and area.
Make a gouge in the wooden stud with your screwdriver for the spacers to fit into. It should be easy to hold in place when you hammer the nails.
These are made for new work in walls. For a ceiling fixture, use a round box with extending arms to nail to the rafters/joists