Here is where you will find a vast array of plastic transition boxes used to finish your installation. Sizes will mate to you supply air vent size.
Sizes and options can be located below.  On the transition boxes, the first portion equals the vent size.  The second portion equals the hose size.  The last portion equals where the hose/ring comes off the transition box.
Example:
TB86-6B = 8″ x 6″ transition box (supply air vent size) with a 6″ ring (ducting to connect here) off the back of the box (“B” ).

1. “TB” = transition box

2. “86” = 8″ x 6″ vent size

3. “6B” = 6″ ring off the back

Example #2:
TB44-4E = 4″ x 4″ transition box (supply air vent size) with a 4″ ring (ducting to connect here) off the end/side.

1. “TB” = transition box

2. “44” = 4″ x 4″ vent size

3. “4E” = 4″ ring off the end/side