If your opening is over 12 inches square and is being used on the floor, then you'll need this solid welded steel framework that will provide support so you won't have to worry about people walking or standing on the grille. There are a lot of vintage houses with very large eggcrate grilles, usually broken. This is the perfect solution! Available on the rounded edges style.
Call for quote, or use this formula: (.74)(duct width)(duct length) + 40.
So, for example, an opening that is 10 by 20 inches would be .74 x 10 x 20+40, which works out to $188. That would include the welded steel and also the eggcrate register.