the best way I ended up was buying the surco mounting brackets they are at very good price only 39 i think for the set of 4 ,this ones mount perfect to the factory rails Not the gutters.
the I got 1 inch sqaur steel tubing to conect them acros welding a little plate to the tubing this little plate rest on top of the brackets with a hole for the bolt, if you dont want to weld the plate you can just put a hole thrue the sqaure tubing but this way you will need the 1.5 inch sqaure tubing.
then you get some plastic caps and paint them
very inexpensive and very eficient and strong,, used them to the huasteca expedition without a single problem or vibrations.
the surco bractes are model j100