This should fit a 2 inch (50.8mm) hole and give you a nice louvered cover. I recommend rotating and printing with louver side down.

If you need a different size, please scale accordingly.

Update: This fit perfectly in a hole, that I cut, with a 2 inch hole saw mounted in a drill.

V1 - louvered circle grill
V2 - louvered circle grill with 1 central support running down the center of the louvers
V2 - louvered circle grill with 2 supports running down the louvers for extra stability.