A/C Flexible Duct, Need Ideas

[hemi8]

I have this hard plastic piece that fits to a Vintage Air evaporator, its about 15" x 3". I need to cut out about a 3" section from the middle and install a flexible section out of rubber or some material to give me some room for adjustment. Any ideas on what to use or how to do it? The only thing I can come up with is taking it to an A/C ductwork shop and letting them install one like in a house, but I would rather do something myself.

Mike