Not sure of the plane you are using it on but I have used a heavy hub inside of a spinner before. It gives you the weight you need and you can still keep the look of the spinner. But, that does depend on the size and shape of the spinner you use.
ORIGINAL: Stickbuilder
Harry Higley also offers prop spinner nuts in different lengths to accomodate your needs. You can get them in different weights to make your balancing needs easier too.
Bill, AMA 4720
WACO Brotherhood #1