Someone already did a laser test across the Bonneville salt flats. I think they went 10/15 miles showing no curvature, what-so-ever.
Citation needed.
The reason the salt flats are flat: Water annually fills the slat lake with water, the salt floats on the water mimicking the water surface, horizontally flat, and continues to do so when the water drains/evaporates.
Except they aren't actually flat. Instead they are level. Big difference.
Also, salt doesn't float on water.
One thing which helps life stay in very cold waters is that as water cools, the solubility of the salt decreases, resulting in it separating into a more concentrated salty solution which sinks and a less salty solution which floats and then freezes creating an insulating layer.
You really don't understand the physics of water do you?
There is a lake in Siberia{Lake Baikal}, that would be perfect for your experiment, several hundred miles long, and is considered the largest flat surface in the world when in freezes over.
Flat or level?
It's on youtube, I'm sure you can think of what to put in their search engine.
So one of the least reliable places in the world with Flat Earthers routinely spreading false information and silencing their critics who point out their lies?
Also, I checked on youtube. Couldn't find it.
The closest I found was a visual test using a camera and a car driving away.
There were issues with mirages, and you still see the car disappearing from the bottom up, but the mirages make it too difficult to confirm anything.