IMO, don't crop the sides at all, as the overscan on most any TV will crop the edge of the frame as it is. Any additional cropping on the sides will probably just cause further loss of the frame when viewing on a television.

As for the top and bottom, I would just crop them as close as possible with a rectangular mask -- nothing fancy. If you're trying to create the most "watchable" presentation, then presenting viewers with a cleanly (albeit overly) cropped picture is preferable to any of the other suggestions, which just seem unnatural to me. Even in a projected environment, you're not seeing graduated (feathered) nor curved edges on your picture. Just my 2 cents.

The project is looking amazing so far! Thanks so much for all of your efforts.