Exhaust leak

I just installed a new x-pipe and catback, and there is a leak where the midpipe connects to the headers. Took it to a couple of muffler shops and was told they couldn't do anything about it. It's not too bad, but I'd like to fix it. Any suggestions?
 
i'm pretty sure there is some kind of putty at most hardware stores you can get for exhaust to seal leaks, think it cures with heat, but is not permanent so you can still get things apart if needed....