You could consider drilling a small hole in the center of the back board of each drawer. Then drill a corresponding hole through the back side of the drawer cabinet unit itself. Remove the drawer and install a piece of heavy string through the hole in the back of the drawer. On the inside if the drawer knot the string so it cannot pull through the hole in the back of the drawer. (You could even tie the string to a small object like a large bead, or small washer). Make this string long enough that it can extend to inside the drawer cabinet when the drawer is fully removed.
Next reach into the back of the drawer cabinet and feed the other end of the string out through the hole in the back of the cabinet. Once pulled through the back hole then install the drawer into its slot and pull the excess string out through the rear of the cabinet. Now calibrate how much slack to leave in the string by pulling the drawer out as far as you think it should be allowed to go and still not pull free from its slot in the cabinet. Mark the string as to where it pulls back through the rear hole when the drawer is extended.
The next step is to secure the string at the marked position so that it cannot pull farther than the mark. This can be achieved in a number of ways as follows.
1) Knot the string at the mark with a large enough knot so it cannot pull back through the rear hole.
2) Tie a small object to the string at the mark position such as a large bead other object.
3) Pinch the end of the string under a washer and screw that is installed into the back if the drawer cabinet.
4) Fill the rear hole with some glue which would go in around the string and permanently affix it into the hole.