I couldn't get a hold of the backplate either, I asked around local mechanical workshops, all quoted me $200++ for making such plate in aluminium.
If you know someone with access to a CNC then maybe you can get it done for a reasonable price.


In my case, I 3D modelled a fan mounting bracket( aka cowl) and got it printed locally ( 3dhubs.com), it costed about $15.
The cowl is attached to the backside of the antminer with duct-tape.
The hose you are referring to is called a duct, sonodec on the other hand is a hose.
The duct is attached to the fan, and the fan to the cowl. M4 screws, nuts and washers can be obtained locally.

So my setup goes like this:
flexible hose -> duct -> exhaust fan -> cowl -> Z9 mini -> intake fan


I can upload some pictures later once I get home.