How was it mounted by the factory?
Double sided "foam"tape will work but depending on how hot the fixture gets it also might fail or make a mess if you ever have to remove the fixture.
Another approach would be rare earth magnets epoxied to the ceiling and fixture, if they are not ferrous. that retains the ability to remove the fixture if needed.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles