The frame for the fans is made up from 1”x1/2”x1/8” aluminium angle. I clamped the fans into two lengths of the angle then marked the mounting holes with an electric drill and 4mm bit. Then I marked where the aluminium obstructed the air flow with a fine OHP pen. I then took the assembly apart and completed the holes using a drill press and the cut outs to clear the air flow using a band saw.
I then screwed the fans in using 4mm screws and nyloc nuts.
In order to get the fans under the radiator, the end supports swivel. The last pair of fixing screws for the fans at each end are reversed so that the excess thread pokes upward and locates the unit between the two panels of the radiator once the end supports are swivelled into their final position.
These screws locate the fan unit under the radiator.