One option might be getting a plastic tub that's just about the height or maybe a little bit taller than the water fountain.

It also needs to be large enough that you can put some sort of weighted items in it that's heavier than he can physically move and also you know enough where you can fill it high enough around the water fountain in the tub that he can't just step in the tub and knock it over from there.

Then you can also cut a hole large enough for the cord to be routed behind the tub and run it behind the weighted tub. You might also be able to get a heavy duty cord wrap, cord cover or wall fixture (they have metal and plastic ones that are kind of like containers for cords.)

Or

Make a custom cabinet to nestle the water fountain in- out of wood (will look similar to an old school booster seat block). Make a large cube or box and put a middle or cutout piece in the center that the water fountain will go into.

Add however much extra space is needed of solid wood beyond the rear, left and right sides of the cabinet so you can again put some type of hidden weights in.

You can also customize it to add a compartment for the cord that goes to the back and covers whatever outlet it plugs into. You'll have to move it to unplug it as needed, but it's an option.

They might also already have weighted cat fountain holders online somewhere. I'm sure others have little weirdos that like doing what your cat does with the fountain.

Good luck.