Narrowing the beam of an ultrasonic sensor

I need to measure the distance to a solid or liquid in a round tank about 2 feet wide and 5 feet deep using a sensor that can output PWM and run at 3.3 or 5 volts.

After testing, I've found that the HC-SR04 has too wide a beam to measure accurately at all distances within the tank, from what I assume is echos off the walls. I've found a MaxBotics sensor that works but is too expensive to use when making multiple sensing units.

Do you guys know of any cheap sensors that may be able to fulfill these requirements or a way to narrow the beam of the HC-SR04? It seems like an "exponential horn" on the sensor is one way to narrow the beam, but I can't find any resources on the dimensions of such a horn.

I'd be interested in seeing the code, data, and a project drawing that lead you to believe that the HC-SR04 won't work in that situation.

But regardless, the HC-SR04 may not live long in a high humidity environment (tank with liquid). For that service, try a JSN-SR04T if the dead zone is acceptable.

Also, Google

ultrasonic distance sensor horn

That gives at least one scientific paper studying horn dimensions.