Okay - that seems to work
As for silicon - it is a practical solution with little expense, and you won't have to get/make something custom (from scratch or customizing another part). Don't know how it would react to body work and/or how it long it will last (UV, heat, vibration, etc.) - possibly someone will know what is suitable. As for look/styling not sure - if black silicon it probably isn't going to draw much attention... other colours might.
While still running with the silicon idea... though it would up expense slightly... maybe see if you can get some black PVC pipe that fits the hole in bumper (I would suspect if it did so reasonably tightly the sensor would sit on top of the other end. Would require less silicone and colour of it probably wouldn't be noticeable - downside is it would require someway to feed the wires thorough the short piece of pipe.