Most secure bed cover?
There reaches a point where it'd be easier to cut a chunk of bedside out I think. 
Mine had a Lo Pro QT roll up on it when I purchased it, I figured a good snow would damage that so I bought a Undercover that you can walk on.
Only time I'm gonna have something valuable enough in there to worry about locking the cover is if I am on the road headed to inlaws or maybe shopping, and then I'll not be leaving it unattended with valuables in there in a motel parking lot or for very long in a store parking lot.
Otherwise, at home or in short rides / drives here, family visits, etc .... and even shopping when nothing is back there the cover is not locked .... but tailgate is locked always and I have a band on the right side hinge. Most valuable item back there is a 15-20 year old $30 (when new) jack and 4 way lug and a box with a few 30 minute safety flares and a couple ratchet straps.

Mine had a Lo Pro QT roll up on it when I purchased it, I figured a good snow would damage that so I bought a Undercover that you can walk on.
Only time I'm gonna have something valuable enough in there to worry about locking the cover is if I am on the road headed to inlaws or maybe shopping, and then I'll not be leaving it unattended with valuables in there in a motel parking lot or for very long in a store parking lot.
Otherwise, at home or in short rides / drives here, family visits, etc .... and even shopping when nothing is back there the cover is not locked .... but tailgate is locked always and I have a band on the right side hinge. Most valuable item back there is a 15-20 year old $30 (when new) jack and 4 way lug and a box with a few 30 minute safety flares and a couple ratchet straps.


