For hitch racks I would recommend stepping up to tray type ones. Much easier to load and the bike won't e dangling around. I have the T2 and it has worked fine. Even held the bikes when I ended up taking an, ahem, slightly rougher than I thought road this summer. It also has an integrated cable lock. Nothing that will stop a thief for long, but it will prevent someone from grabbing the bike when you stop for lunch.
Roof racks, harder to put the bike on them. If your Flex doesn't have cross-bars already those can get pricy (a couple hundred bucks for towers plus at least a hundred for the bars themselves). If it does have cross-bars I'd recommend Rocky Mounts. Inexpensive and fit almost all types of cross-bars. I've had two of those for almost ten years and they work great. The skewers lock, which again will not stop a determined thief but will prevent an opportunist from taking the bike.
Regardless of which option you go with, you can always throw a cable lock to secure the bikes from casual theft.