Dometic CFX3 35 is a good size.
You can either fit lashing points into the Cali Floor rails and then feed a roof rack strap around the fridge handles / to lashing points (then you can can still open the lid!).
Or you can get a purpose built bracket:
Made from black hex birch plywood This product is suitable for any small/medium sized cooler box (maximum size = 600mm x 380mm It is pictured with an Alpicool G22 Simply attach the cradle to the sliding rails and secure the fridge/cooler with the included strap Cleats included for sliding rails...
www.vangear.co.uk
Ours sits on sliding boot tray under the Multiflex Board, using the roof rack strap method which is fed under the boot tray, and then around the fridge handles.