Yup totally agree with the above.
With my TrakPower 4900's it is close to the belt/pulleys but no sign whatsoever of any marks that would suggest it's been rubbing even with my "special" style of driving
So in theory as long as it fits under, you don't need to worry about any flex. So in theory any packs without the bumps should do you fine.
(TrakPower ftw though 
).