I recently installed a Truck Covers USA American Roll Cover on my 6.5' bed Duramax. It has the hard aluminum slats and they advertise it as having the smallest footprint canister on the market. I chose it over all the other roll covers for several reasons, but one of which is that it DOES NOT have a latch to open on the top (only a hard mounted key slot). It does have a pull-cable latch on the underside at the tailgate, which is the only way I've ever used to open it.
I was steered away from the Roll-N-Lock because I read reviews stating that all you to do was jump on it in order to dislodge it from the canister and create access to the underneath.
Oh, and it DOES NOT leak.