I would imagine, just two clear plastic containers, water, salt, and a freezer. A ice cube tray would also probably work, putting saltwater in half of the holes and normal water in the other half. Put it in the freezer and then check on it every 5 minutes or so to see how frozen they are.
I would use two zip-lock bags, gallon size, and put the two types of water into them, seal them up, and stick them in the freezer.
For the salt water, I'd use approximately the same salinity as sea water, which varies but is about 3.5%. So, using a quart of water, which is 32 fluid ounces and (since it's water) 32 ounces by weight, too. The means you need to add about 1.12 ounces (3.5% of 32) of salt. I just weighed that out and it's about 4 and 3/4 teaspoons of salt.
Put the bags in the freezer and check them every half hour to assess how the freezing is coming along.
You may find that the salt water does not freeze at all. If your freezer as at 32 degrees Fahrenheit, then the salt water won't freeze. Actually, it's the water in the salt water that freezes, leaving the salt behind, and so the salinity of the remaining water goes up over time. The chemical processes at the boundary of the frozen water and salt water are such that you'll have to drop the temperature of your freezer several degrees (say, 10 degrees) to get much ice in the salt water bag.