Don't have experience with mounting hi-lift, but for long trip I would either use something solid like this one
http://www.amazon.ca/Hi-Lift-Jack-TM.../dp/B003CEODQI or mount it inside the trunk with bungees. Standard bungees can't go under roof rails (maybe there are ones which can, but I didn't see them) so you might want to consider ratchet straps with bungees. Reliable mounting hi-lift even in ABS pipe might be complicated. Last thing you want is your hi-lift flying off the roof on a highway. Besides on the roof it will be loud. Just my 2 cents, maybe if you ask on 5gen forum you'll get more ideas : )