What, if anything can you attach to a beam?
Beams are designed to support “Live Loads” and “Dead Loads”.
All materials that are applied to a beam will make them deflect. If you use material like gypsum board it may crack, because it’s not designed to deflect. If you use wood trim or siding, it won’t crack because it’s made to deflect (somewhat).
If you don’t know the design or the live and dead loads accounted for in the design, I’d use some 3/4” thick wood trim and stain to match other trim in the room.