You need a temperature well below that of plain melting ice. If you don't have a freezer, or don't want to use one, this could be achieved by making what is known as a freezing mixture. In its simplest form this can be made by mixing crushed ice and salt (table salt, NaCl). Chilling both in the freezer beforehand might help to get it as cold as possible. The trouble is, this will still slowly warm up again unless the mixture is replenished, but you have to be able to do that without disturbing your naphtha. Perhaps you would be able to get around this by using a lot of freezing mixture or, if you can, substituting calcium chloride hexahydrate and snow for the ice and salt, which would get considerably colder.