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I think there use to be something available to make small repairs (nicks or cracks) to urethane, but I don't know of anything suitable to fill urethane bumpers to fit them to a body.
I always try to talk my customers out of urethane bumpers if they're just replacing the bumper, and not painting the car at the same time. With a fiberglass bumper, you can sand, grind or fill the bumper to have it fit the body. If the urethane doesn't fit, your pretty much into doing body work on the car.
Are you using a new or used OEM bumper or do you have the $199.00 aftermarket one. The $199.00 replacement one is known for being difficult to get it to fit properly.
I wish I could offer you something more positive. Maybe someone else on the forum knows of a better solution.
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.