Narrowing the beam of an ultrasonic sensor
Since your specs are just hand-waving arguments, I can suggest a really small Satelite parabolic dish with an offset sensor.
But for giggles, I would try a long heat-shrunk tubing over the sensor to attenuate the stray angular signals.
But this path loss will also attenuate the sensing range so refocusing all the energy makes more sense. (no pun intended) Focal path dispersion will occur from irregular shapes and result in weaker noisy edges.
Spec: effectual angle: less than 15°; distance: 2cm~500cm
It might be tried to measure the acoustic resonant frequency of the remaining air in the tank using a low-level sweep generator to an acoustically flat external speaker air plenum. (like a bass reflex effect)Then calibrate.
Another possible solution is plastic coated electrodes for capacitance or SRF frequency measurements.
Yet another is using UHF to measure the return loss in a splitter to a diode detector and FET buffered mV reading. Sweep the RF to detect the wavelngth for either a null detection  or a peak half wave. Then calibrate for accuracy. The antenna could be a short rubber ducky, that does not need to be even unity gain as antenna efficiency drops with length.
But if you are trying to make a $1 sensor, you got your work cut out for you.