Great idea, Manpa!
Whether you use bottles (l use two liter milk bottles) or those PVC tubes, fill them up with brine. You can't drink them, but the salt makes the ice freeze at a lower temperature.
A 23% solution of brine by weight freezes at -21.1 C.
Since most domestic freezers don't get below 18C, I'd aim at about a 20% solution of salt by weight, which works out at about 200g per liter of water, or 400g of salt in two litres of water.
That solution should freeze at -16.7C, much colder than plain water at 0C.
Note that you should make the brine in a pot on the stove (to facilitate dissolution) and then top up your container, as putting the salt into the plastic container will result in a stronger solution, as the salt will displace some of the volume of water.
It's a good idea to top up your containers to about 80% to allow for expansion on freezing.
Bigger containers are better than small ones, as they have a greater thermal density and will thaw slower. You need to find a balance, though, to make sure you have enough room for your stuff.