If you opt for the cheaper (relative) canvas bag vs the hardtop, someone can just slash the fabric and run away with your stuff.
The hardtop is much safer but expensive, the lock is also very funky.
You are looking at around 60K and up for a Thule system.
P.S. You need a special permit to use roof mounted carryons. I believe the term is "Top Load" on your LTO permit. Tsikot members have been harassed by MMDA in the past.