Well, if money is no object, here is EXACTLY what you are looking for:
Foldable Cart For Pak-o-Bird (Small and larger size with13" width)
But it is very spendy! The cart alone is $125 and the carrier depends on the size, and whether you get stainless steel mesh. I just came back from a winning trip to Vegas, so I splurged and ordered a medium customized carrier for my CAG!