Add black (or other color) padding when doing vstack (or hstack)
First, setup a color filter of the same height, and then add it to the hstack

-filter_complex "color=black:5x240[c];[v1][c][v2]hstack=inputs=3:shortest=1[videoout]"


See the options for the color filter here.



Alternate method is

-filter_complex "[0]pad=iw+5:ih:color=black[0v];[0v][v2]hstack[videoout]"