I often times have 3-4 pelican cases on my roof. I've used a bunch of different ratcheting straps over the years and these turned out to be great.
Husky 16 ft. x 1-1/2 in. Ratchet Tie-Downs (2-Pack)-FH0942 - The Home Depot
I also have a nice long cable lock to secure my gear onto the roof from a snatch and grab. Easy to weave though the case handle of inside of the lock shackle as I run extended shackles.