Select the two duplicate shapes.

From the Arrange menu, select Create Bitmap Copy. Leave the Color Depth: setting at True Color + Alpha to create a bitmap with a transparent background.

In the Bitmap Size tabbed section, change the DPI: setting to 200. Press Create to create the bitmap.

Even though 96 dpi is screen resolution and generally good enough for a bitmap of this kind, 200 dpi will retain more detail that we can scale up or down if need be.

Center the bitmap we just created over the other elements.

Select the bitmap, select the Transparency Tool, and apply a Flat, Stained Glass transparency.

Wow. Did that license plate just age, or what?