I did a different AC mod 2 years ago, and I also had a hard time finding the correct size air duct parts back then. I ended up buying this hose from Amazon:
However, that one is currently unavailable, but I now see several 2.5" hoses on Amazon. Some examples:
Mounting flange:
I'm planning on doing the same mod myself (it seems far superior), so I would be interested in hearing about your experience with it. Both building it, and how well the mod works.
Roy B