I use a Topeak rack trunk and even though it's not "hard" like the other bags previously suggested, it does it's job very well and can be quickly removed and attached to the rack with the cool quick locking function that Topeak uses. Using a hard sided trunk for "protection" probably won't work too well in my opinion though as a thief would more likely just steal the entire bike then just break open the truck open at their own convinience--plus a noticable trunk might attract a curious thief as well. Personally I'd rather just be able to quickly remove the trunk and take it with me.