I see someone has posted about pancake boxes already. They also make "U" shaped boxes that fit on either side of the joist, and a one-sided version if you aren't centered. I like these as they are strong and give you more volume for the wires.

https://www.homedepot.com/p/Legrand-Pass-Seymour-Slater-New-Work-Plastic-4-In-Round-16-Cu-In-Direct-Mount-Fixture-Fan-Ceiling-Box-S116FAN/304270956

https://www.homedepot.com/p/Commercial-Electric-12-cu-in-Plastic-NM-Fan-Box-with-Plastic-Cover-CPB13NM-SM/205383211