We use INNO storage box and the bottom is flat for most of it. They also use M8 bolts to lock them onto the T slot rack. My wife has a Thule rack on her car. I have not put it on the FR but I did come up with a solution. Get yourself 4 wooden blocks that sit just above the FR rack's outer rim. Drill holes to fit an M8 bolt through the four blocks. Bolt box to rack. I'm not saying you can do 100 but you should be able to do 65.

Just a suggestion and like I said, I have not tried it yet. We use a ski rack from INNO that I have bolted to the FR rack.