Hi, I use a :Thule Sonic Cargo Box, no issues, very simple to use, I basically use it to carry 2 big inflatables kayaks and all the gear (lifejackets, etc.), after using the kayaks didnt want to get all the sand and salt water inside the jeep..and not a single drop ever gets out of the box so i think that if no water comes out then there shouldnt be any water coming in right ?.
To me its been a great solution for my needs.
Rgds. and here is a pic so you can see what it looks like on top of my Jeep