Zip ties would be quick and easy, but it would kill the airflow through the radiator. Up close, the fans would only blow through the rad fins where the blades are...not at the hub, and not at the fan's frame or anywhere else...lots of waisted cooling space.
You need to build a shroud to get the fans at least one inch away from the surface of the radiator...that way the airflow will even out over all the fins of the rad.
I stole this pic from one of DodgeViper's posts, as he did an excellent job, and it shows fan orientation well. You can make it out of brass as he did, copper as Hoot did, or aluminum as I did. You can also use cardboard, cake pans, shoeboxes, tupperware, or any number of materials to get the fans away from the core's surface, and provide a decent seal against the radiator.