Well, unless I misunderstand your question, you actually just stated your comp settings precisely.
2048×1556 (mask for 2048×858)
Which is actually an AE comp preset, Film 2K.
That’s it, pretty much. Just add mask or masked solid for letterboxing, the borders of which would be…let’s see…(1556-858)/2 = um, 349 pixels in from the top and bottom edge of the comp, it seems.
Did they tell you what delivery format they require?