How can i fit a rear silencer that is too big at the joint?

I recently bought a new rear silencer from ebay but when i've come to fit it, it is blowing at the joint. Is there any way to remedy this? I'm not sure if the exhaust that was already fitted is a standard one for my car, but the new rear silencer i bought seems to be a little big at the joint. I've never replaced one before so don't know if there is some sort of filler to use or other. Can anyone help?