This is the one I got: https://www.amazon.com/MaxMate-Tri-Fold-2002-2018-2003-2018-Fleetside/dp/B00REJXVEW/ref=mp_s_a_1_3?keywords=maxmate+tonneau+tri+fold&qid=1551726803&s=gateway&sr=8-3
It’s not lockable - and it’s vinyl - so someone can cut through it. But if you lock your tailgate, a curious person can’t just lift it - they have to break it to get in. It is water tight (at least my stuff doesn’t get wet in rain)
It’s probably not what you’re looking for - but I more wanted to recommend the portability it offers. It takes 2 min to take it completely off - which I’ve done often.
There are higher quality / more expensive ones that do the same.