Find some wood and attach it

Use the screws that came with the drawer slides. The wood can be longer than the bracket. Mine's a full 24 inches long and reaches all the way to the back of the cabinet.

That's just three-quarter inch plywood that I had lying about. I used my table saw to cut the proper dimensions. You can pick up little squares of plywood at Home Depot and they'll even cut them for you for 50 cents a cut.