take a spacer and a cog and put it behind the biggest cog in the cluster. So it doesn't look odd, put the extra cog flat against cassette.

all shimano did when they went from 7 speed to 8 speed was to lengthen the cassette body by a cog+spacer. spacing is the same, so you can run 8 speed shifters on 7 speed cogsets, you just have an extra click. i do it all the time with repairs and sharpie out the 8 on the shifter.