Convert Micro exhaust vent to outside

[Red Roadrunner]

We have the over the range microwave with hood vent that exhaust to inside our fifth wheel. We want to vent it to the outside.The unit is in the slide out next to the refrigerator. We do not have the installation manual or template. The manual is not available to order or down load online for this specific Magic Chef model. I down loaded the manual for a generic MC model. We have removed the microwave and determined that the wall mounting plate has a pre-cut out area for wall exhaust duct and the fan can be moved for the exhaust out the back instead of the top. The issue we have is the only exhaust ducts we have been able to find is designed for 3.25" x 10" rectangular duct or round duct. We do not have room for round duct adapter, and the pre-cut wall mount opening and the duct opening on the microwave is 2.25" x 8". Anyone know solution to this issue?

[Falconman]

I would just use the larger vent and make my own transition out of some flashing.

[Bama Rambler]

I agree, just cut the opening to accommodate the duct and then use high temp flue tape to seal it off to the hood.