Attaching poorly-fitting exhaust components without welding?

I recently replaced my muffler, and attached it to the existing pipe using clamps around a short coupler pipe (sorry, I don’t know the right name for these things) that fits around the outside the the pipe attached to the muffler and the existing pipe under the vehicle. I wasn’t able to get a straight (same diameter all the way) coupler around the existing pipe, so I chose one that was slightly larger at the end that goes around the exhaust pipe, and rather than fitting perfectly snug, the coupler buckled and exposed a gap (and exhaust leak) when I tightened the clamp down on it.

I’m pretty sure to fix it properly I’m going to have to rip the coupler pipe back off (probably with my Dremel) and replace it with something different, but I’m not sure how to determine the right parts I should be using? Do I just need to sand down the (moderately rusted on the outside, but still intact) exhaust pipe under the car and try to get the smaller coupler pipe around it? Any good resource for beginners doing exhaust repair work like this?

I can take some pics or pipe measurements if it would help.