The L1X Regions of Interest (ROI) are a great idea, but the L1X is primarily a long-distance chip and might give you the wrong answer at the very, very short distances. So test that. If you can back the sensor into the body of your housing by a couple of centimeters, you will be fine.
Other solutions:
Calibrate the container. Light hitting the sides and returning will lead to a wrong answer, but that wrong answer should be consistent. So with a little calibration of your contain you should be able to match the ranging result to the actual fullness.
Salt is really reflective. If you paint the walls of the container black, the number of photons from the walls will be overwhelmed by those from the salt and give you the right answer. Might not work if the salt sticks to the sides of the container though.
I have also found that building a cap over the Transmit side of the sensor and putting a small hole in it will block the light from spreading. The trick is to do the geometry to calculate the size of the hole and the thickness of the cap. But the cap has to be optically black in the IR region - which takes some work. (Black rubber works well.) And of course you have to completely block any stray light from hitting the cap and bouncing back immediately.
But one has to admit that using the VL53L1 and ROIs is a pretty easy solution.
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