With your limited means/space, have you considered using the internet? There are some excellent free sites that help improve English vocabulary. My favorite is Vocabulary.com. Your students can quickly and easily make an account and start being tested on words. At first they will only be asked about easy words, for instance the word "tether".
Vocabulary.com is great for ESL students because its definitions are simple and intuitive, and most of them are accompanied by a memorable little blurb.
Tell me if this works for you,