angle bracket narrow enough to fit in the bracket you bought, and then bend it over the rack tube.

how wide is the channel in the bought bracket? Plumber's strap might work... loop it over the rack and then attach it again on the backside...

A Star Washer between the bracket and rack might work, but won't be a great solution...

zip ties in the right places might work...

and some handwork with a Rat tail file might make the bracket conform to the Rack's top tube, as "a pair of tabs"

i'd try the strap over the tube first... any hardware store will have plumbers strap material... it might need trimmed to width, or might have too large of holes in it... use the channel on the backside of the light mount to trap the strap... double up the strap to add strength...
That same hardware store will have Brass, Steel, and aluminum sheets that could be used to make an L-bracket to hook over the top rail and set in the channel on the back of your new mount bar... you'll need some tin snips and a drill/motor. A 2x4 will work as a form to bend the L-bracket...