1/2" strip of metal, 1/8" thick. drill and countersink two holes to mount it the res. add two Hose clamps in the SAE size ranges of 26 to 28 to clamp the res to the metal strip. bend the bottom of the metal strip 90 degrees under the res if you want to get fancy.
if you have res clamps with outside holes you can just get proper thread/length screws. no need for a bracket. this example is Alphacool Cape Cyclone (available in 60/80mm).