Problem with ultrasonic sensor

I have a ultrasonic sensor (HC-SR04) and i have a problem when I want to measure a distance from the sensor to a flat surface, when the flat surface isn't perpendicular to the sensor wave direction. The point is that the wave deviates and it doesn't return to the sensor or the distance mesured is wrong (too high).
Here I put a sketch so you can understand it easier.

Line 2 is the flat surface that isn't perpendicular. The ultrasonic problem (red line) is the bounce of the wave.

I would like to give me some idea to fix that problem. I mean there may be other sensor's for distance mesuring that wouldn't have this problem.

Thank you.

Multiple sensors, angled either side of the existing one?

i want a sensor to replace the existing one. The same case, the same angle, just change the sensor.