Defroster hose attaching?

Does anyone know what diameter hose and how to attach it to the defroster duct below the cowl on my 59 cj5 with factory driverside fresh air heater setup? The defroster duct below the cowl has a inside diameter of 2.5 x 2.75 so I don't know which hose to buy to connect it to the plastic reproduction walcks4wd.com heater plenum that has a 2 inch outlet and a 1.5 inch outlet.

I called walcks and they said I needed to step up the size of the holes on their heater plenum to a larger size, im just not sure how much it should be stepped up. Right now im thinking of bringing it up to 2.5 inches and using hose clamp on bottom and stuffing it inside heater duct and using duct tape to secure the top??? any other ideas?

 

I might check out Fernco couplings at the hardware store. They will adapt to about anything.

<http://www.fernco.com/plumbing/flexible-couplings/stock-couplings>

 

I used the 2.5" defrost hose and inserted it into the plastic reproduction defrost duct. Then I found some metal clips to hold the duct from coming out of the plastic duct.

 

I tried to get a picture. Not very good being that it was taken from under the dash. If you look close. It's right by the pink wire. It is a "S" shaped clip, kind of gold in color. I used three of them.