Is the problem that the ends of the new muffler bracket aren't curved down enough to contain the rubber hangers? Try using a long heavy duty(3/8'wide)HVAC zip tie. Run it through both the rubber hangers on each side, reach in w/ long needle nose and cinch it up to your desired stretch point, then cut the excess. As long as it isn't touching the muffler, it won't melt, promise. It is amazing how strong these ties are. It's not a pretty fix(but who sees down there), and may work for you. Your muffler shop sucks if they can't figure out a proper way to secure it. The bracket may need to be modified, welded,bent, curved?? But you definitely want rubber isolaters of some sort there.