How should I splice the very short wires for this LED shop light into my 12/2 NM-B?
What you want is a ceiling box that uses an old-work brace

If you have ever been in a hardware/home-improvement store and wondered why some of the boxes had what looked like a long metal brace attached to them, like so (picture for illustration only):



wonder no further.  That brace is meant to go between adjacent ceiling joists and support both the box and a fixture attached to the box, and the box can be adjusted to position it in the joist bay.  The fixture itself then acts as the cover for the box, as per NEC 314.25 and 410.22.

(Trying to support a box using a lighting fixture would likely be seen as a NEC 314.23 violation -- there is no subsection of 314.23 that allows for that.)