1-Gang 20 cu. in. New Work Electrical Ceiling Box with Captive Nails

There are a few things which can be used to mimic recessed lights but you wouldn’t have the ability to use most retro fit LED trims. Perhaps something like the below from Cree? It would allow you to use traditional mount screws into the gang with the look of “almost” recessed. If you want true recessed light, you will need traditional recessed cans or flush mount wafers. Cree 6 in. White Integrated LED Recessed Disk Light Trim https://www.homedepot.com/p/304591815

TL;DR: Use a high quality 8-32 machine screw. I did some more research on this after posting, and managed to answer my own question in more detail. Thanks to Thomas & Betts and Greg for your responses! I found some online debates about what type of screws were appropriate, and war stories about run-ins with electrical inspectors, but I eventually found an Engineering Bulletin from NEMA (National Electrical Manufacturers Association) that addresses this specifically: Bulletin No. 109, Screw Attachments to Electrical Boxes - Guidelines for Screw Substitution, Approved Feb. 5, 2015. The short(er) version of that article is: * Use a high quality machine screw of the appropriate size and 32 threads per inch. * Never use drywall screws! * The screw bosses in plastic covers may crack or break if you use screws that are too large or with different threading, and temperature fluctuations may also cause the plastic to fail. * 6-32 and 8-32 are the most common screw sizes, but follow manufacturer instructions. * In the case of a metal box (though this one is plastic), it is important to have the correct threads (32 per inch) to make sufficient contact with the electrical box threads for electrical bonding between the box and a metallic cover plate or canopy on your light fixture, in addition to the threads providing mechanical security (i.e. your fixture doesn't fall off the wall). I also read elsewhere that a big reason drywall screws should be avoided is that they are not rated for shear strength, and can easily break off due to the cheap materials used to make them. Electrical code requires that you only use materials designed for their purpose, which drywall screws with no shear rating fail for this purpose. While the screw bosses on this fixture are the appropriate diameter for an 8-32 machine screw, they are NOT pre-threaded, so you likely have to use either an 8-32 tap to thread the plastic, or a bit of elbow grease to force the screw to thread into the plastic.

Although rated for up to 50lb, these are not rated/listed for ceiling fans and most building inspectors will make you swap them out for ones meant for a ceiling fan. Per code: Where a box is used as the sole support for a ceiling paddle fan, the box must be listed for this purpose and include the weight of the fan. Ceiling paddle fans not in excess of 35 lb. can be supported directly to an outlet box that is identified for this purpose [422.18(A)]. Ceiling paddle fans exceeding 35 lb. must be supported independently of the outlet box, unless the box is listed to support the weight of the fan [422.18(B)]. So things don’t pull loose in a few years, you would be better off with (or something else which is specifically for ceiling fans): 25 cu. in. Ceiling Fan Box - Super Blue https://www.homedepot.com/p/100173618

They can. I have used 3” deck screws.