Are you thinking like a standard 8 string with pairs? If so, you might be able to use a 5 and omit the tuner reams and bridge routing. I imagine the tuner reams would be tight on the headstock.
If you're thinking 8 like taking a 6 and adding an additional low and high string....whoa... That would take a custom neck and neck pocket. $$$$ there. I tried working out a narrow 7 using a 6 but found the strings would come off the end of the neck, even with the narrowest string spacing I could fine. (@ 16.5-ish mm)