Loud PSD exhaust

[The Duck]

Hi guys,
I wonder if some of you could point me in the right direction?
I have one of the PSD exhausts (very similar to the current PPP version) with a straight through centre and back box.
Now I don't know if I am getting old but I want to make the exhaust a little quieter. Would anyone know who made the exhaust and where they are at?

I am tempted to either get a larger middle box or a non-straight through back box.

Thanks

Andrew