Surface mount troffer on ceiling
so you plan to get 2 of 4' pieces of C channel and screw it to your ceiling 2' apart then slide the troffer in from one end.



not to scale 

possibly you could use two of these drop ceiling edge moldings
 riveted together to form a C

Else you could get a sheet-metal place to fold C channel for you, or see what you can find in aluminum extrusions.