Most of the convection/microwaves are made so they can be set up to either recirculate or exhaust externally (I've installed several of them). You could get the external exhaust kit, move the existing baffle plate (or make a new one if necessary), see if there are any frame members in the cut-out area, and convert yours to external exhaust. I gave a broken one to my son-in law. It was set up for external venting. He repaired the oven and installed it in their house which was not set up for external venting. He had no problem switching the oven from external back to internal. It's just a matter of cover up one hole vs. another.
Rob