So I think I have fixed at least this design to work for any line input. Although it is bulky. Shout out to ssilk for the starting design.
Note: |---| is the direction the underground belt is running. | | is the opening.

upper lane:
> > 1/4 #---#\
> > 0 ---- #\
>##### > > 1/2 #-# > > 1/4 ...