The louvers on our microwave exhaust vent were breaking down from years of UV exposure, and rather than completely remove the cover thought to just replace the louvers with a simple 3d print.  A UV resistant and somewhat flexible filament is suggested; we opted for PETG for now as it was on hand, may replace with ASA or something else if they don't hold up.  These fit our approximately 8 inch by 8 inch vent, you may want to modify to better fit your vent.

We had good luck printing in PETG with the following settings (can add more if someone's interested):

• 100% infill
• 0.12 layer height
• mouse ears or brim

Because this is wide and flat, you may want to monitor for the first few layers.

The louvers need enough mass to fall back down after flipping up from running the vent fan, hence the 100% infill. The small layer height helps keep the end tabs round. A brim or at least mouse ears are suggested (we used mouse ears with good results, though we're printing in an enclosed chamber).  Also watch minimum layer times, as the final layers print quickly.

Depending on your layer height and/or the quality of your print, you may want to lightly sand/smooth the end tabs to ensure the louvers rotate freely when installed.