Firstly, you will need a large plastic storage box or tote. If you do not have one already they are inexpensive to buy. A clear box is preferable so that you can easily see inside to view your hamster, but they can be hard to find so you may have to settle for a semi-transparent box. The sides of the box should be tall and smooth so that the cage will be secure. Any protruding plastic could be chewed by the hamster and could result in it escaping the box, so be careful to check the inside walls of the box. A box with handles will make it easier to carry when necessary.
The box will need to be sanitised before using it as a cage to ensure that it does not cause your hamster to become ill. You can do this by washing it with soapy water and a small amount of bleach. Once you have cleaned all round the box you should rinse it and dry it thoroughly.