I've welded 2 brackets to support top of radiator to the horizontal rail...... but they flex a bit - maybe 5 mm if I push quite hard.

the steel is 1 inch box section cut to make U-shape and thinner steel U in the rear to make it box.

Will the flex be problem?

The radiator rests on a rail and there will be rubber grommets (the L shaped peice on LHS is for mock up) that should abosrb some movement.
Rescued attachment DSC05931.JPG

It will be a problem eventually. I'd use something a bit thicker, i'm using 4x15mm strip to support mine now as my original ones failed after 9k.

If you wanted to save weight on a full tube, then it would have made more sense to cut it down at a diagonal IMO

... not so much weight saving but I cannot go full width of rail due to brake line going on the top.

Also think that it may cause some problems later on with fatigue!
Here is a pic of my brackets, rock solid and out of 1.5mm ally....

Rad and fan