A 60mm od collar was fabricated and bonded to the duct housing. During fabrication the part was trial fitted and heater hose cut and rough fitted.
The part was cleaned with soapy water to remove all dirt and grime, then cleaned with grease and wax remover, sanded with 320 grit sand paper, cleaned again with grease and wax remover and primed and painted with 500 degree engine enamel.
The process took about 6 hours not including curing and drying time for fiberglass and paint. I installed part about 2 weeks ago and I have been enjoying cabin heat. It does not blow heat as if the original factory equipment was installed but enough to warm the cabin.
I can think of a modification to this design that might deliver more efficient heat. I am going to purchase another factory heat duct, modify the same and incorporate the modification and let you know the results. Stay tuned.