I would mount links of chain to the front of the horizontal tube, cut open to make a hook, positioned vertically. Then mount a length of chain off top L and R corners of each gong and hook the chains to the bar via the hooks. Have the chains angle outwards away from the gong so they dont twist and turn when hit. Use nice hot tacks to attach everything, poor fusion will lead to eventual failures. Then when its time to pack up you can unhook the gongs, you could also make different gongs and switch them out as you like.