Exhaust hanger advice

I recently hung a full 2.5" system with X-pipe under my Dart, with exit just ahead of the axle. I used one rubber dog-bone hanger per muffler. Works great on OEM, right?

Wrong...the dogbones have rotted in two, and my system is laying on the ground. It's all welded together until the collectors, which is my fault, so it's a PITA to mount new dogbones, but I'm wondering, what did I do wrong?

Do I need to use several (more than a pair, total) of them? The system isn't that heavy (so it seems) and I didn't drive the car that much, but these things are d-r-y rotted. Thoughts? Ideas? I am actually debating making a hanger out of heim joints to allow the system to move, but I don't know about how the vibes would transfer.