What tripod length? I can fit my spotting scopes and tripod from Precision Rifle Solutions inside my packs, no need to hang the tripod off the outside.
But if you must, like was said by someone else, run the legs under compression straps, snug them up to hold it on. Not sure if that pack has true compression straps so if not, I prefer lashing/webbing straps with buckle, holds skis, tools, tent poles a whole host of kit on outside of my pack. Another option is bungee cord with toggles. Velcro straps I find poor in wet and cold, freezes up and fails to stick but it will work.
If your tripod is something like 3/4 gear used in production companies with camera work, big and heavy, I laid it horizontal on the top of the main pack compartment under the top lid and lashed it down with several webbing straps. The issue is, it stuck out a great amount on each side and would hit and stick on trees and brush, but due to its length and weight, had to be carried that way.