You have the right idea, but needs to be reinforced. Even though the material may be the same/similar as the substrate it is still weaker overall. I recommend using fiberglass matting as reinforcement within the filler, moreso than a metal mesh. Not only will allow more flex without promoting cracks, but in the event you need to drill or grind through it, will be considerably more forgiving.
Just use the same materials to bond the mat. Do not use fiberglass resin on a urethane bumper ;-)