I reused tin work from my propane system and fabricated an extension for the cold air boot
It houses two blower motors like the poster above. I used strips of fiber glass at the seams to seal it and also make it accessible for motor maintenance. Its not pretty lol, but it all works good. The boot holds a 16 x 24 inch filter. I decided to try one of the washable reusable ones. I take it out every 3 weeks and clean it. Its paid for itself with the first few washings.