I want to create a register/ return air type box about 4" high so my bathroom fan's exhaust will be above the joists so that the duct can have the shortest and most direct run to a gable vent. Is there a good pre-made product that would work to line it, or should I just line a box made of 2x4s with some galvanized steel sheet?
I would install 2x blocking to mimic how it would mount normally and then create a chute using rectangular duct.
Rectangular duct is available, pre bent with 1 bend, you buy 2 to assemble. You can buy a slightly larger size and just tape the overlap to get a custom size. Be sure to use a metallic tape and not ordinary "duct" tape.
Make your chute several inches longer (than needed) and cut down along the corner seams to be able to bend out 1" mounting flanges on each end.
You may need to make some mounting ears to attach the grille. Perhaps a pair of screws left proud of the chute would catch the spring loaded bail (bale?).