2013 Toyota Aygo exhaust back box

[Albertf47]

Good morning all, I’m wondering if you could offer some advice? I’m literally brand new to this forum. I’ve recently bought my daughter her first car. A 2013Toyota Aygo Ice. I absolutely love it. I’ve completed all the maintenance myself. I’ve changed everything from the engine Oil to the Brake fluid. I’m trying to say that I’m reasonably competent with car maintenance. 

However, I wanted to replace the rear exhaust back box as the old one looked worst for wear and started to rot in the usual places. I took the old one off and fitted the new one.
 

I can not for the life of me get it to seal onto the existing straight pipe. I’ve used exhaust paste and a new original style bracket. There’s still a blowing leak every time from the joint. Am I doing something wrong? The back box was an after market part on eBay and seemed absolutely fine but now I’m doubting that aperture may be out of tolerance with original straight pipe (if indeed it is  the original straight pipe). I can’t send it back, in my infinite wisdom, I sprayed it black with high temp, rust resistance paint in an effort to expand it usable life. 
 

can anyone offer me some advice? 
 

Thanks in advace

Edited 2 hours ago by Albertf47
Missed words